diff --git a/tests/unit/test_ai_avatar_render_service.py b/tests/unit/test_ai_avatar_render_service.py
index 62cf82691..99e28928b 100644
--- a/tests/unit/test_ai_avatar_render_service.py
+++ b/tests/unit/test_ai_avatar_render_service.py
@@ -547,7 +547,7 @@ class TestAiAvatarRenderService:
         with (
             patch.object(svc, "_download_video", return_value="/tmp/video.mp4"),
             patch.object(svc, "_upload_to_oss", side_effect=lambda path, key: f"https://oss/{key}"),
-            patch("os.system", return_value=0),
+            patch("subprocess.run") as mock_run,
             patch("tempfile.TemporaryDirectory") as tmpdir_mock,
             patch(
                 "app.services.ai_avatar_cover_service.generate_smart_cover", return_value="https://oss/smart_cover.jpg"
@@ -557,6 +557,9 @@ class TestAiAvatarRenderService:
                 "packages.adapters.sqlalchemy_impl.generated_video_repository.SQLAlchemyGeneratedVideoRepository"
             ) as repo_cls,
         ):
+            import subprocess as _sp
+
+            mock_run.return_value = _sp.CompletedProcess(args=[], returncode=0, stdout="", stderr="")
             import tempfile as _tf
 
             tmpdir_mock.return_value.__enter__ = MagicMock(return_value="/tmp/testdir")
@@ -605,10 +608,13 @@ class TestAiAvatarRenderService:
         with (
             patch.object(svc, "_download_video", return_value="/tmp/video.mp4"),
             patch.object(svc, "_upload_to_oss", side_effect=lambda path, key: f"https://oss/{key}"),
-            patch("os.system", return_value=0),
+            patch("subprocess.run") as mock_run,
             patch("tempfile.TemporaryDirectory") as tmpdir_mock,
             patch("app.services.ai_avatar_cover_service.generate_smart_cover", side_effect=RuntimeError("DB error")),
         ):
+            import subprocess as _sp
+
+            mock_run.return_value = _sp.CompletedProcess(args=[], returncode=0, stdout="", stderr="")
             tmpdir_mock.return_value.__enter__ = MagicMock(return_value="/tmp/testdir")
             tmpdir_mock.return_value.__exit__ = MagicMock(return_value=False)
             svc.execute_render("render-clip-fail")
diff --git a/tests/unit/test_lipsync_speed_optimization.py b/tests/unit/test_lipsync_speed_optimization.py
index 7c4a39eb3..5963b8a51 100644
--- a/tests/unit/test_lipsync_speed_optimization.py
+++ b/tests/unit/test_lipsync_speed_optimization.py
@@ -35,7 +35,10 @@ class TestFFmpegPresetOptimization:
             final_label=None,
             output_path="/tmp/output.mp4",
         )
-        assert "-preset veryfast" in cmd, f"期望 -preset veryfast，实际命令: {cmd}"
+        # cmd 现在是 list[str]；preset 与值是相邻两个元素
+        assert "-preset" in cmd, f"期望包含 -preset，实际命令: {cmd}"
+        preset_idx = cmd.index("-preset")
+        assert cmd[preset_idx + 1] == "veryfast", f"期望 veryfast，实际: {cmd}"
 
     def test_preset_veryfast_with_filter(self):
         """带滤镜场景下也必须使用 veryfast."""
@@ -49,7 +52,8 @@ class TestFFmpegPresetOptimization:
             final_label="[v]",
             output_path="/tmp/output.mp4",
         )
-        assert "-preset veryfast" in cmd
+        assert "-preset" in cmd
+        assert cmd[cmd.index("-preset") + 1] == "veryfast"
         assert "-filter_complex" in cmd
 
     def test_preset_not_fast(self):
@@ -65,11 +69,11 @@ class TestFFmpegPresetOptimization:
             output_path="/tmp/output.mp4",
         )
         # 确保是 veryfast 而不是 fast
-        assert "-preset veryfast" in cmd
-        # 排除 "fast" 单独出现（veryfast 包含 fast 子串，需精确判断）
-        parts = cmd.split()
-        preset_idx = parts.index("-preset")
-        assert parts[preset_idx + 1] == "veryfast"
+        assert "-preset" in cmd
+        preset_idx = cmd.index("-preset")
+        assert cmd[preset_idx + 1] == "veryfast"
+        # 禁止 fast 单独作为 preset 值（veryfast 包含 "fast" 子串，不影响）
+        assert cmd[preset_idx + 1] != "fast"
 
 
 # ═══════════════════════════════════════════════════════════════════════════════