diff --git a/unilabos/layout_optimizer/obb.py b/unilabos/layout_optimizer/obb.py
index eaf0a6b2..e89619a1 100644
--- a/unilabos/layout_optimizer/obb.py
+++ b/unilabos/layout_optimizer/obb.py
@@ -183,6 +183,56 @@ def _point_in_convex(
     return True
 
 
+def segment_obb_intersection_length(
+    p1: tuple[float, float],
+    p2: tuple[float, float],
+    corners: list[tuple[float, float]],
+) -> float:
+    """线段 p1-p2 与 OBB（凸多边形）的交集长度。
+
+    Cyrus-Beck 线段裁剪算法。corners 假定为 CCW 顺序（obb_corners 生成）。
+    无交集返回 0.0。
+    """
+    dx = p2[0] - p1[0]
+    dy = p2[1] - p1[1]
+    seg_len_sq = dx * dx + dy * dy
+    if seg_len_sq < 1e-24:
+        return 0.0
+
+    t_enter = 0.0
+    t_exit = 1.0
+    n = len(corners)
+
+    for i in range(n):
+        ax, ay = corners[i]
+        bx, by = corners[(i + 1) % n]
+        # CCW 多边形边的外法线: (ey, -ex), e = b - a
+        ex, ey = bx - ax, by - ay
+        nx, ny = ey, -ex
+
+        denom = nx * dx + ny * dy
+        numer = nx * (p1[0] - ax) + ny * (p1[1] - ay)
+
+        if abs(denom) < 1e-12:
+            if numer > 0:
+                return 0.0  # 在此边外侧且平行
+            continue
+
+        t = -numer / denom
+        if denom < 0:
+            t_enter = max(t_enter, t)   # 进入
+        else:
+            t_exit = min(t_exit, t)     # 退出
+
+        if t_enter > t_exit:
+            return 0.0
+
+    if t_enter >= t_exit:
+        return 0.0
+
+    return (t_exit - t_enter) * math.sqrt(seg_len_sq)
+
+
 def obb_min_distance(
     corners_a: list[tuple[float, float]],
     corners_b: list[tuple[float, float]],
diff --git a/unilabos/layout_optimizer/tests/test_obb.py b/unilabos/layout_optimizer/tests/test_obb.py
index 9fc4d455..c3bbe9e8 100644
--- a/unilabos/layout_optimizer/tests/test_obb.py
+++ b/unilabos/layout_optimizer/tests/test_obb.py
@@ -1,7 +1,7 @@
 """Tests for OBB (Oriented Bounding Box) geometry utilities."""
 import math
 import pytest
-from ..obb import obb_corners, obb_overlap, obb_min_distance
+from ..obb import obb_corners, obb_overlap, obb_min_distance, segment_obb_intersection_length
 
 
 class TestObbCorners:
@@ -115,3 +115,42 @@ class TestObbMinDistance:
         a = obb_corners(0, 0, 2.0, 2.0, 0.0)
         b = obb_corners(2.0, 0, 2.0, 2.0, 0.0)
         assert obb_min_distance(a, b) == pytest.approx(0.0)
+
+
+class TestSegmentOBBIntersectionLength:
+    """segment_obb_intersection_length: Cyrus-Beck clipping."""
+
+    def test_segment_fully_outside(self):
+        corners = obb_corners(0, 0, 2, 2, 0)
+        length = segment_obb_intersection_length((-5, 3), (5, 3), corners)
+        assert length == 0.0
+
+    def test_segment_fully_inside(self):
+        corners = obb_corners(0, 0, 4, 4, 0)
+        length = segment_obb_intersection_length((-0.5, 0), (0.5, 0), corners)
+        assert abs(length - 1.0) < 1e-6
+
+    def test_segment_crosses_through(self):
+        corners = obb_corners(0, 0, 2, 2, 0)
+        length = segment_obb_intersection_length((-5, 0), (5, 0), corners)
+        assert abs(length - 2.0) < 1e-6
+
+    def test_segment_partial_overlap(self):
+        corners = obb_corners(0, 0, 2, 2, 0)
+        length = segment_obb_intersection_length((0, 0), (5, 0), corners)
+        assert abs(length - 1.0) < 1e-6
+
+    def test_rotated_obb(self):
+        corners = obb_corners(0, 0, 2, 2, math.pi / 4)
+        length = segment_obb_intersection_length((-3, 0), (3, 0), corners)
+        expected = 2 * math.sqrt(2)
+        assert abs(length - expected) < 1e-4
+
+    def test_zero_length_segment(self):
+        corners = obb_corners(0, 0, 2, 2, 0)
+        assert segment_obb_intersection_length((0, 0), (0, 0), corners) == 0.0
+
+    def test_parallel_outside(self):
+        corners = obb_corners(0, 0, 2, 2, 0)
+        length = segment_obb_intersection_length((-5, 2), (5, 2), corners)
+        assert length == 0.0