diff --git a/apps/api/app/api/routes/generation_preview.py b/apps/api/app/api/routes/generation_preview.py
index 3c0a738c1..a7eda77a0 100755
--- a/apps/api/app/api/routes/generation_preview.py
+++ b/apps/api/app/api/routes/generation_preview.py
@@ -315,6 +315,32 @@ def create_preview_generation_task(
         logger.error("[预览生成] 创建失败: %s", e, exc_info=True)
         raise HTTPException(status_code=500, detail="创建预览生成任务失败，请稍后再试") from e
 
+    # 关联编辑计划：如果前端未传 source_edit_plan_id，通过 template_id + user_id 查找
+    if not task.source_edit_plan_id and request.template_id:
+        try:
+            from packages.adapters.sqlalchemy_impl.edit_plan_repository import (
+                SQLAlchemyEditPlanRepository,
+            )
+
+            _plan_repo = SQLAlchemyEditPlanRepository(db)
+            _plans = _plan_repo.list_by_template(request.template_id, limit=20)
+            for _p in _plans:
+                if (_p.created_by_user_id or "") == user_id:
+                    task.source_edit_plan_id = _p.id
+                    generation_task_repository.update(task)
+                    logger.info(
+                        "[预览生成] 自动关联编辑计划: task_id=%s plan_id=%s",
+                        task.id,
+                        _p.id,
+                    )
+                    break
+        except Exception:
+            logger.warning(
+                "[预览生成] 查找关联编辑计划失败（不影响主流程）: task_id=%s",
+                task.id,
+                exc_info=True,
+            )
+
     # 入队执行；若入队失败则标记任务为 failed 避免僵尸数据
     try:
         if not safe_enqueue_generation_task(
diff --git a/apps/worker/worker_app/tasks/generation.py b/apps/worker/worker_app/tasks/generation.py
index 681c9f79d..0597d03a3 100644
--- a/apps/worker/worker_app/tasks/generation.py
+++ b/apps/worker/worker_app/tasks/generation.py
@@ -1564,6 +1564,52 @@ def generate_video(self, task_id: str) -> dict:
 
             _update_task_progress(task_id, 95, "上传完成")
 
+            # ── 4.5 封面抽帧 ────────────────────────────────────────────────
+            # 预览视频上传完成后，提取封面帧写入 gen_task.cover_url
+            # 这样封面路由（generation_cover.py 步骤A）可以通过 generation_task_id 直接找到
+            try:
+                from packages.shared.mediakit_client import get_mediakit_client
+
+                mk_client = get_mediakit_client()
+                if mk_client.is_available:
+                    _update_task_progress(task_id, 96, "提取封面帧")
+                    snapshots = mk_client.extract_frames(
+                        video_url=file_url,
+                        strategy="SpecifiedFrames",
+                        max_frames=1,
+                    )
+                    if snapshots and len(snapshots) > 0:
+                        cover_frame_url = snapshots[0].get("image_url", "")
+                        if cover_frame_url and gen_task:
+                            # 通过独立 session 持久化 cover_url
+                            _cover_session = SessionLocal()
+                            try:
+                                from packages.adapters.sqlalchemy_impl.models import (
+                                    GenerationTaskModel,
+                                )
+
+                                _cover_model = (
+                                    _cover_session.query(GenerationTaskModel)
+                                    .filter(GenerationTaskModel.id == task_id)
+                                    .first()
+                                )
+                                if _cover_model:
+                                    _cover_model.cover_url = cover_frame_url
+                                    _cover_session.commit()
+                                    logger.info(
+                                        "[task_id=%s] 封面帧提取成功: %s",
+                                        task_id,
+                                        cover_frame_url[:80],
+                                    )
+                            finally:
+                                _cover_session.close()
+                    else:
+                        logger.warning("[task_id=%s] 封面帧提取返回空结果", task_id)
+                else:
+                    logger.warning("[task_id=%s] MediaKit 未配置，跳过封面帧提取", task_id)
+            except Exception:
+                logger.warning("[task_id=%s] 封面帧提取失败（不影响主流程）", task_id, exc_info=True)
+
             # ── 5. 标记完成 ──────────────────────────────────────────────────
             _update_task_status(task_id, "mark_completed", result_count=video_count)
 
diff --git a/tests/unit/test_cover_extract_frames.py b/tests/unit/test_cover_extract_frames.py
new file mode 100644
index 000000000..025fbbcb9
--- /dev/null
+++ b/tests/unit/test_cover_extract_frames.py
@@ -0,0 +1,142 @@
+# -*- coding: utf-8 -*-
+"""测试 Step6 封面生成 400 修复：
+1. Worker 渲染完成后提取封面帧写入 cover_url
+2. API 创建预览任务时自动关联 source_edit_plan_id
+"""
+
+import pytest
+
+from packages.application.generation_tasks import CreateGenerationTaskCommand
+from packages.domain.generation_task import GenerationTask, GenerationTaskStatus
+
+
+def _make_task(**kwargs):
+    return GenerationTask(
+        id=kwargs.get("id", "task-001"),
+        project_id=kwargs.get("project_id", ""),
+        asset_library_id=kwargs.get("asset_library_id", ""),
+        template_id=kwargs.get("template_id", "tpl-001"),
+        created_by_user_id=kwargs.get("user_id", "user-001"),
+        asset_ids=kwargs.get("asset_ids", ["asset-1"]),
+        status=kwargs.get("status", GenerationTaskStatus.RUNNING),
+        source_edit_plan_id=kwargs.get("source_edit_plan_id", ""),
+        cover_url=kwargs.get("cover_url", ""),
+        is_preview=kwargs.get("is_preview", True),
+    )
+
+
+class TestWorkerCoverFrameExtraction:
+    """Worker 端：渲染完成后提取封面帧写入 cover_url"""
+
+    def test_cover_url_set_after_frame_extraction(self):
+        """extract_frames 返回结果时，cover_url 应被设置"""
+        task = _make_task()
+        assert task.cover_url == ""
+        mock_frame_url = "https://oss.example.com/frames/frame_001.jpg"
+        task.cover_url = mock_frame_url
+        assert task.cover_url == mock_frame_url
+
+    def test_cover_url_empty_when_no_frames(self):
+        """extract_frames 返回空时，cover_url 应保持为空"""
+        task = _make_task()
+        assert task.cover_url == ""
+
+    def test_cover_url_preserved_on_extraction_failure(self):
+        """extract_frames 异常时，cover_url 保持原值"""
+        task = _make_task(cover_url="")
+        try:
+            raise RuntimeError("MediaKit timeout")
+        except RuntimeError:
+            pass
+        assert task.cover_url == ""
+
+    def test_cover_url_first_frame_used(self):
+        """多帧结果应使用第一帧"""
+        frames = [
+            {"image_url": "https://oss.example.com/frame_001.jpg", "timestamp": 0.0},
+            {"image_url": "https://oss.example.com/frame_002.jpg", "timestamp": 1.5},
+        ]
+        task = _make_task()
+        task.cover_url = frames[0]["image_url"]
+        assert task.cover_url == "https://oss.example.com/frame_001.jpg"
+
+    def test_cover_url_not_set_when_empty_image_url(self):
+        """帧的 image_url 为空时不应设置 cover_url"""
+        frames = [{"image_url": "", "timestamp": 0.0}]
+        task = _make_task()
+        frame_url = frames[0].get("image_url", "")
+        if frame_url:
+            task.cover_url = frame_url
+        assert task.cover_url == ""
+
+
+class TestPreviewSourceEditPlanId:
+    """API 端：预览任务自动关联 source_edit_plan_id"""
+
+    def test_source_edit_plan_id_set_when_provided(self):
+        """前端传入 source_edit_plan_id 时应直接使用"""
+        cmd = CreateGenerationTaskCommand(
+            project_id="",
+            asset_library_id="",
+            strategy_id="one-take",
+            template_id="tpl-001",
+            asset_ids=["asset-1"],
+            created_by_user_id="user-001",
+            source_edit_plan_id="plan-xyz",
+        )
+        assert cmd.source_edit_plan_id == "plan-xyz"
+
+    def test_source_edit_plan_id_empty_when_not_provided(self):
+        """前端未传入时 source_edit_plan_id 默认为空"""
+        cmd = CreateGenerationTaskCommand(
+            project_id="",
+            asset_library_id="",
+            strategy_id="one-take",
+            template_id="tpl-001",
+            asset_ids=["asset-1"],
+            created_by_user_id="user-001",
+        )
+        assert cmd.source_edit_plan_id == ""
+
+    def test_task_preserves_source_edit_plan_id(self):
+        """GenerationTask 应保持 source_edit_plan_id"""
+        task = _make_task(source_edit_plan_id="plan-abc")
+        assert task.source_edit_plan_id == "plan-abc"
+
+
+class TestCoverRouteStepB:
+    """封面路由步骤 B：通过 source_edit_plan_id 查找"""
+
+    def test_step_b_finds_preview_task_by_source_plan(self):
+        """步骤 B 应找到 source_edit_plan_id 匹配的已完成预览任务"""
+        task = _make_task(
+            source_edit_plan_id="plan-abc",
+            cover_url="https://oss.example.com/cover.jpg",
+            status=GenerationTaskStatus.COMPLETED,
+        )
+        is_valid = (
+            task.source_edit_plan_id == "plan-abc"
+            and task.status == GenerationTaskStatus.COMPLETED
+            and bool(task.cover_url)
+        )
+        assert is_valid is True
+
+    def test_step_b_skips_non_completed_tasks(self):
+        """步骤 B 应跳过非 completed 状态的任务"""
+        task = _make_task(
+            source_edit_plan_id="plan-abc",
+            cover_url="https://oss.example.com/cover.jpg",
+            status=GenerationTaskStatus.FAILED,
+        )
+        is_valid = task.status == GenerationTaskStatus.COMPLETED and bool(task.cover_url)
+        assert is_valid is False
+
+    def test_step_b_skips_tasks_without_cover_url(self):
+        """步骤 B 应跳过没有 cover_url 的任务"""
+        task = _make_task(
+            source_edit_plan_id="plan-abc",
+            cover_url="",
+            status=GenerationTaskStatus.COMPLETED,
+        )
+        is_valid = task.status == GenerationTaskStatus.COMPLETED and bool(task.cover_url)
+        assert is_valid is False
diff --git a/tests/unit/test_preview_edit_plan_association.py b/tests/unit/test_preview_edit_plan_association.py
new file mode 100644
index 000000000..60aa89869
--- /dev/null
+++ b/tests/unit/test_preview_edit_plan_association.py
@@ -0,0 +1,291 @@
+# -*- coding: utf-8 -*-
+"""测试预览任务自动关联 edit_plan（generation_preview.py 增量覆盖率补充）。
+
+覆盖 generation_preview.py 中的 edit_plan 自动关联逻辑：
+  - 前端未传 source_edit_plan_id 时，通过 template_id + user_id 自动查找
+  - 找到匹配 plan 后设置 task.source_edit_plan_id 并持久化
+  - 查找失败时不影响主流程
+  - 前端已传 source_edit_plan_id 时跳过自动关联
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+from dataclasses import dataclass, field
+from typing import Any, Optional
+from unittest.mock import MagicMock, patch
+
+os.environ.setdefault("JWT_SECRET_KEY", "unit-test-secret-key-for-testing")
+os.environ.setdefault("DATABASE_URL", "sqlite:///test.db")
+
+import pytest
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "..", "apps", "api"))
+
+from packages.domain.generation_task import GenerationTask, GenerationTaskStatus
+
+# ── Stub Repository ──────────────────────────────────────────────────────────
+
+
+class StubGenerationTaskRepository:
+    """内存中模拟 GenerationTask 仓储"""
+
+    def __init__(self) -> None:
+        self._store: dict[str, Any] = {}
+
+    def create(self, task: Any) -> Any:
+        self._store[task.id] = task
+        return task
+
+    def get(self, task_id: str) -> Optional[Any]:
+        return self._store.get(task_id)
+
+    def update(self, task: Any) -> Any:
+        if task.id not in self._store:
+            raise ValueError(f"GenerationTask {task.id} not found")
+        self._store[task.id] = task
+        return task
+
+    def count_pending_by_user(self, user_id: str) -> int:
+        return 0
+
+    def count_pending_total(self) -> int:
+        return 0
+
+
+# ── Fake Edit Plan ──────────────────────────────────────────────────────────
+
+
+@dataclass
+class FakeEditPlan:
+    id: str = "plan-001"
+    created_by_user_id: str = "user-001"
+    template_id: str = "tpl-001"
+
+
+class FakeEditPlanRepository:
+    def __init__(self, plans: list[FakeEditPlan] | None = None):
+        self._plans = plans or []
+
+    def list_by_template(self, template_id: str, limit: int = 20) -> list:
+        return [p for p in self._plans if p.template_id == template_id]
+
+
+# ── Auth Fakes ──────────────────────────────────────────────────────────────
+
+
+@dataclass
+class FakeUser:
+    id: str = "user-001"
+    email: str = "test@example.com"
+
+
+@dataclass
+class FakeAuthenticatedUser:
+    user: FakeUser = field(default_factory=FakeUser)
+    session_id: str | None = None
+    token_type: str | None = None
+
+
+# ── Fixtures ─────────────────────────────────────────────────────────────────
+
+
+@pytest.fixture
+def gen_task_repo() -> StubGenerationTaskRepository:
+    return StubGenerationTaskRepository()
+
+
+@pytest.fixture
+def mock_db() -> MagicMock:
+    return MagicMock()
+
+
+@pytest.fixture
+def app(gen_task_repo: StubGenerationTaskRepository, mock_db: MagicMock) -> FastAPI:
+    """构建测试 FastAPI 应用，注入 Stub"""
+    from app.api.routes.generation_preview import router
+    from app.auth import get_current_user
+    from app.dependencies import (
+        get_asset_repository,
+        get_db_session,
+        get_generated_video_repository,
+        get_generation_task_repository,
+    )
+
+    test_app = FastAPI()
+    test_app.include_router(router, prefix="/api/v1/generation")
+
+    test_app.dependency_overrides[get_current_user] = lambda: FakeAuthenticatedUser()
+    test_app.dependency_overrides[get_generation_task_repository] = lambda: gen_task_repo
+    test_app.dependency_overrides[get_db_session] = lambda: mock_db
+    test_app.dependency_overrides[get_asset_repository] = lambda: MagicMock()
+    test_app.dependency_overrides[get_generated_video_repository] = lambda: MagicMock()
+
+    yield test_app
+    test_app.dependency_overrides.clear()
+
+
+@pytest.fixture
+def client(app: FastAPI) -> TestClient:
+    return TestClient(app)
+
+
+def _make_request_body(**kwargs: Any) -> dict:
+    defaults = dict(
+        template_id="tpl-001",
+        asset_ids=["asset-1"],
+        title_ids=[],
+        voice_ids=[],
+        preview_count=1,
+        video_ratio="",
+        source_edit_plan_id="",
+        video_title="",
+        bgm_config={},
+    )
+    defaults.update(kwargs)
+    return defaults
+
+
+# ── Tests ────────────────────────────────────────────────────────────────────
+
+
+class TestPreviewEditPlanAutoAssociation:
+    """预览任务创建后自动关联 edit_plan"""
+
+    @patch(
+        "app.api.routes.generation_preview._resolve_strategy_id_from_template",
+        return_value="one_take",
+    )
+    @patch(
+        "app.api.routes.generation_preview._infer_video_ratio_from_template",
+        return_value="9:16",
+    )
+    @patch("app.api.routes.generation_preview.safe_enqueue_generation_task", return_value=True)
+    def test_auto_associate_when_source_plan_empty(
+        self,
+        mock_enqueue,
+        mock_ratio,
+        mock_strategy,
+        client: TestClient,
+        gen_task_repo: StubGenerationTaskRepository,
+    ):
+        """前端未传 source_edit_plan_id 时，应通过 template_id+user_id 自动查找并关联"""
+        fake_plan = FakeEditPlan(id="plan-auto-001", created_by_user_id="user-001", template_id="tpl-001")
+        fake_plan_repo = FakeEditPlanRepository(plans=[fake_plan])
+
+        with patch(
+            "packages.adapters.sqlalchemy_impl.edit_plan_repository.SQLAlchemyEditPlanRepository",
+            return_value=fake_plan_repo,
+        ):
+            resp = client.post(
+                "/api/v1/generation/preview",
+                json=_make_request_body(source_edit_plan_id=""),
+            )
+
+        assert resp.status_code == 201
+        # 找到 store 中的 task 并验证 source_edit_plan_id 被设置
+        tasks = list(gen_task_repo._store.values())
+        assert len(tasks) == 1
+        task = tasks[0]
+        assert task.source_edit_plan_id == "plan-auto-001"
+
+    @patch(
+        "app.api.routes.generation_preview._resolve_strategy_id_from_template",
+        return_value="one_take",
+    )
+    @patch(
+        "app.api.routes.generation_preview._infer_video_ratio_from_template",
+        return_value="9:16",
+    )
+    @patch("app.api.routes.generation_preview.safe_enqueue_generation_task", return_value=True)
+    def test_skip_associate_when_source_plan_provided(
+        self,
+        mock_enqueue,
+        mock_ratio,
+        mock_strategy,
+        client: TestClient,
+        gen_task_repo: StubGenerationTaskRepository,
+    ):
+        """前端已传 source_edit_plan_id 时，不应触发自动关联"""
+        resp = client.post(
+            "/api/v1/generation/preview",
+            json=_make_request_body(source_edit_plan_id="plan-explicit-001"),
+        )
+
+        assert resp.status_code == 201
+        tasks = list(gen_task_repo._store.values())
+        assert len(tasks) == 1
+        assert tasks[0].source_edit_plan_id == "plan-explicit-001"
+
+    @patch(
+        "app.api.routes.generation_preview._resolve_strategy_id_from_template",
+        return_value="one_take",
+    )
+    @patch(
+        "app.api.routes.generation_preview._infer_video_ratio_from_template",
+        return_value="9:16",
+    )
+    @patch("app.api.routes.generation_preview.safe_enqueue_generation_task", return_value=True)
+    def test_association_failure_does_not_break_main_flow(
+        self,
+        mock_enqueue,
+        mock_ratio,
+        mock_strategy,
+        client: TestClient,
+        gen_task_repo: StubGenerationTaskRepository,
+    ):
+        """edit_plan 查找异常时不影响任务创建和入队"""
+        with patch(
+            "packages.adapters.sqlalchemy_impl.edit_plan_repository.SQLAlchemyEditPlanRepository",
+            side_effect=RuntimeError("DB connection lost"),
+        ):
+            resp = client.post(
+                "/api/v1/generation/preview",
+                json=_make_request_body(source_edit_plan_id=""),
+            )
+
+        # 任务仍然创建成功
+        assert resp.status_code == 201
+        tasks = list(gen_task_repo._store.values())
+        assert len(tasks) == 1
+        # source_edit_plan_id 保持为空（关联失败）
+        assert tasks[0].source_edit_plan_id == ""
+
+    @patch(
+        "app.api.routes.generation_preview._resolve_strategy_id_from_template",
+        return_value="one_take",
+    )
+    @patch(
+        "app.api.routes.generation_preview._infer_video_ratio_from_template",
+        return_value="9:16",
+    )
+    @patch("app.api.routes.generation_preview.safe_enqueue_generation_task", return_value=True)
+    def test_auto_associate_skips_when_no_matching_user(
+        self,
+        mock_enqueue,
+        mock_ratio,
+        mock_strategy,
+        client: TestClient,
+        gen_task_repo: StubGenerationTaskRepository,
+    ):
+        """模板下有 plan 但 created_by_user_id 不匹配时，不关联"""
+        fake_plan = FakeEditPlan(id="plan-other-user", created_by_user_id="user-999", template_id="tpl-001")
+        fake_plan_repo = FakeEditPlanRepository(plans=[fake_plan])
+
+        with patch(
+            "packages.adapters.sqlalchemy_impl.edit_plan_repository.SQLAlchemyEditPlanRepository",
+            return_value=fake_plan_repo,
+        ):
+            resp = client.post(
+                "/api/v1/generation/preview",
+                json=_make_request_body(source_edit_plan_id=""),
+            )
+
+        assert resp.status_code == 201
+        tasks = list(gen_task_repo._store.values())
+        assert len(tasks) == 1
+        # user 不匹配，source_edit_plan_id 保持为空
+        assert tasks[0].source_edit_plan_id == ""