diff --git a/packages/domain/ass_subtitle_builder.py b/packages/domain/ass_subtitle_builder.py
index cacbf1e6d..ed4d2733f 100755
--- a/packages/domain/ass_subtitle_builder.py
+++ b/packages/domain/ass_subtitle_builder.py
@@ -35,6 +35,17 @@ FONT_NAME_MAP: dict[str, str] = {
     "华康俪金黑": "Noto Sans SC",
 }
 
+# ASS Fontsize 是字体 em-square 高度（含 Latin 升降部留白），
+# 中文字符实际只占声明字号的约 65%~75%；浏览器 CSS font-size 让中文字符占满声明高度。
+# 为让成片中文字高与前端 CSS 预览一致，写入 ASS 时对字号乘以补偿系数。
+# font_size=89 → ASS Fontsize=round(89*1.35)=120，实际中文字高约 78~85px。
+ASS_FONTSIZE_COMPENSATION = 1.35
+
+
+def _compensate_ass_fontsize(font_size: int) -> int:
+    """将 CSS 语义字号换算为 ASS Fontsize，补偿中文字符在 em-square 中的留白。"""
+    return max(1, round(font_size * ASS_FONTSIZE_COMPENSATION))
+
 
 # ── 颜色转换 ──────────────────────────────────────────────────────────────────
 
@@ -137,8 +148,11 @@ def build_ass_style(
     # Shadow 深度：shadow_offset[1] 作为纵向偏移
     shadow_depth = shadow_offset[1] if shadow_blur > 0 else 0
 
+    # 写入 ASS Style 时对字号做补偿，使成片中文字高与前端 CSS 预览一致
+    ass_font_size = _compensate_ass_fontsize(font_size)
+
     return (
-        f"Style: {style_name},{actual_font},{font_size},{primary_color},"
+        f"Style: {style_name},{actual_font},{ass_font_size},{primary_color},"
         f"&H000000FF,{outline_color},{back_color},"
         f"{bold_val},{italic_val},0,0,100,100,0,0,"
         f"1,{outline_width},{shadow_depth},{alignment},"
@@ -210,6 +224,9 @@ def _wrap_title_text(
     if available_width <= 0:
         return text
 
+    # 换行宽度必须与实际渲染（补偿后的 ASS Fontsize）一致，否则换行位置会错位
+    ass_font_size = _compensate_ass_fontsize(font_size)
+
     # 先按已有 \N 分段，每段独立自动换行，最后用 \N 拼回
     segments = text.split("\\N")
     wrapped_segments: list[str] = []
@@ -221,7 +238,7 @@ def _wrap_title_text(
 
         for ch in seg:
             # CJK 字符按全角估算，其他按半角
-            char_width = float(font_size) if ord(ch) > 0x2E80 else font_size * 0.55
+            char_width = float(ass_font_size) if ord(ch) > 0x2E80 else ass_font_size * 0.55
 
             if current_width + char_width > available_width and current_line:
                 lines.append(current_line)
diff --git a/tests/unit/domain/test_ass_subtitle_builder.py b/tests/unit/domain/test_ass_subtitle_builder.py
index 1a7913983..a9a9c8e68 100644
--- a/tests/unit/domain/test_ass_subtitle_builder.py
+++ b/tests/unit/domain/test_ass_subtitle_builder.py
@@ -105,9 +105,9 @@ class TestBuildAssStyle:
 
     def test_contains_font_size(self):
         result = build_ass_style("S1", font_size=36)
-        # Style行格式：Name, Fontname, Fontsize, ...
+        # Style行格式：Name, Fontname, Fontsize, ...（36*1.35=48.6→49）
         parts = result.split(",")
-        assert parts[2] == "36"
+        assert parts[2] == "49"
 
     def test_bold_true(self):
         result = build_ass_style("S1", bold=True)
@@ -420,7 +420,7 @@ class TestBuildAssContent:
         for line in result.split("\n"):
             if line.startswith("Style: TitleStyle"):
                 parts = line.split(",")
-                assert parts[2] == "72"
+                assert parts[2] == "97"  # 72*1.35=97.2→97
                 break
 
     def test_title_font_size_frontend_field_alias(self):
@@ -435,7 +435,7 @@ class TestBuildAssContent:
         for line in result.split("\n"):
             if line.startswith("Style: TitleStyle"):
                 parts = line.split(",")
-                assert parts[2] == "48"
+                assert parts[2] == "65"  # 48*1.35=64.8→65
                 break
 
     def test_title_font_color_frontend_field_alias(self):
@@ -462,7 +462,7 @@ class TestBuildAssContent:
         for line in result.split("\n"):
             if line.startswith("Style: TitleStyle"):
                 parts = line.split(",")
-                assert parts[2] == "56"
+                assert parts[2] == "76"  # 56*1.35=75.6→76
                 break
 
     def test_title_bold(self):
diff --git a/tests/unit/test_ass_subtitle_builder.py b/tests/unit/test_ass_subtitle_builder.py
index dfa65a68f..9483e298c 100755
--- a/tests/unit/test_ass_subtitle_builder.py
+++ b/tests/unit/test_ass_subtitle_builder.py
@@ -80,11 +80,11 @@ class TestBuildAssStyle:
         result = build_ass_style("TestStyle")
         assert result.startswith("Style: TestStyle,")
         assert "Noto Sans SC" in result
-        assert ",48," in result
+        assert ",65," in result  # 48*1.35=64.8→65
 
     def test_custom_font_size(self):
         result = build_ass_style("Title", font_size=64)
-        assert ",64," in result
+        assert ",86," in result  # 64*1.35=86.4→86
 
     def test_bold_enabled(self):
         result = build_ass_style("BoldStyle", bold=True)
@@ -570,3 +570,35 @@ class TestWrapTitleText:
                 break
         else:
             pytest.fail("未找到 TitleStyle Dialogue 行")
+
+
+class TestFontsizeCompensation:
+    """ASS Fontsize 补偿系数（CSS 字号 → ASS em-square 字号）。"""
+
+    def test_default_48_compensated_to_65(self):
+        result = build_ass_style("S")
+        parts = result.split(",")
+        assert parts[2] == "65"  # round(48*1.35)=65
+
+    def test_89_compensated_to_120(self):
+        """实测对齐点：font_size=89 → ASS Fontsize=120。"""
+        result = build_ass_style("S", font_size=89)
+        parts = result.split(",")
+        assert parts[2] == "120"
+
+    def test_subtitle_also_compensated(self):
+        content = build_ass_content(
+            video_width=1080,
+            video_height=1920,
+            video_duration=5.0,
+            subtitle_text="字幕",
+            subtitle_config={"size": 24},
+        )
+        sub_line = [line for line in content.splitlines() if line.startswith("Style: SubtitleStyle")][0]
+        fields = [f.strip() for f in sub_line.split(",")]
+        assert fields[2] == "32"  # round(24*1.35)=32
+
+    def test_minimum_fontsize_at_least_one(self):
+        result = build_ass_style("S", font_size=0)
+        parts = result.split(",")
+        assert int(parts[2]) >= 1
diff --git a/tests/unit/test_render_subtitles.py b/tests/unit/test_render_subtitles.py
index a445b4130..c0304e986 100755
--- a/tests/unit/test_render_subtitles.py
+++ b/tests/unit/test_render_subtitles.py
@@ -81,12 +81,12 @@ class TestBuildAssStyle:
         style = _build_ass_style("Default")
         assert style.startswith("Style: Default,")
         assert "Noto Sans SC" in style
-        assert "48" in style  # font_size
+        assert "65" in style  # font_size 48*1.35=65
 
     def test_custom_font(self):
         style = _build_ass_style("Custom", font_name="Arial", font_size=32)
         assert "Arial" in style
-        assert ",32," in style
+        assert ",43," in style  # font_size 32*1.35=43
 
     def test_bold(self):
         style = _build_ass_style("Bold", bold=True)
diff --git a/tests/unit/test_render_subtitles_pure.py b/tests/unit/test_render_subtitles_pure.py
index 1322460f7..4f73aac49 100644
--- a/tests/unit/test_render_subtitles_pure.py
+++ b/tests/unit/test_render_subtitles_pure.py
@@ -85,7 +85,7 @@ class TestBuildAssStyle:
         line = _build_ass_style("Default")
         assert line.startswith("Style: Default,")
         assert "Noto Sans SC" in line
-        assert "48" in line  # font_size
+        assert "65" in line  # font_size 48*1.35=65
 
     def test_bold_enabled(self):
         """加粗时Bold=-1."""
@@ -112,7 +112,7 @@ class TestBuildAssStyle:
         """自定义字号."""
         line = _build_ass_style("Big", font_size=72)
         parts = line.split(",")
-        assert parts[2] == "72"  # Fontsize
+        assert parts[2] == "97"  # Fontsize 72*1.35=97
 
     def test_custom_alignment(self):
         """自定义对齐方式."""
diff --git a/tests/unit/test_title_render_consistency.py b/tests/unit/test_title_render_consistency.py
index 246822a8c..99aeaa3c9 100644
--- a/tests/unit/test_title_render_consistency.py
+++ b/tests/unit/test_title_render_consistency.py
@@ -31,7 +31,7 @@ class TestFontSize:
             title_text="测试标题",
             title_config=config,
         )
-        assert ",36," in content, f"默认字号应为36，实际内容: {content}"
+        assert ",49," in content, f"默认字号36应补偿为49(36*1.35)，实际内容: {content}"
 
     def test_size_32_preserved(self):
         """size=32 应原样使用。"""
@@ -43,7 +43,7 @@ class TestFontSize:
             title_text="测试标题",
             title_config=config,
         )
-        assert ",32," in content
+        assert ",43," in content  # 32*1.35=43
 
     def test_size_60_preserved(self):
         """size=60 应原样保留（字号上限已移除）。"""
@@ -58,7 +58,7 @@ class TestFontSize:
         style_line = [line for line in content.splitlines() if line.startswith("Style: TitleStyle")][0]
         fields = [f.strip() for f in style_line.split(",")]
         font_size = int(fields[2])
-        assert font_size == 60, f"字号60应原样保留, 实际={font_size}"
+        assert font_size == 81, f"字号60应补偿为81(60*1.35), 实际={font_size}"
 
     def test_font_size_alias_normalized(self):
         """前端传 font_size 应归一化为 size。"""
@@ -72,7 +72,7 @@ class TestFontSize:
         )
         style_line = [line for line in content.splitlines() if line.startswith("Style: TitleStyle")][0]
         fields = [f.strip() for f in style_line.split(",")]
-        assert fields[2] == "52", f"font_size=52 应归一化, 实际={fields[2]}"
+        assert fields[2] == "70", f"font_size=52 应补偿为70(52*1.35), 实际={fields[2]}"
 
     def test_font_color_alias_normalized(self):
         """前端传 font_color 应归一化为 color。"""
@@ -97,7 +97,7 @@ class TestFontSize:
             title_text="测试标题",
             title_config=config,
         )
-        assert ",24," in content
+        assert ",32," in content  # 24*1.35=32.4→32
 
 
 class TestBooleanStrokeNormalization:
@@ -231,8 +231,8 @@ class TestFullStyleConsistency:
 
         # Fontname
         assert fields[1] == "Noto Sans SC"
-        # Fontsize = 28
-        assert fields[2] == "28"
+        # Fontsize = 28*1.35=37.8→38
+        assert fields[2] == "38"
         # Bold = -1 (True)
         assert fields[7] == "-1"
         # Outline width = 2 (前端默认 stroke width)