diff --git a/primitive_process/interface/subface/simple/extrude_polyline_side_face.hpp b/primitive_process/interface/subface/simple/extrude_polyline_side_face.hpp
index b7b94a3..d0c39e6 100644
--- a/primitive_process/interface/subface/simple/extrude_polyline_side_face.hpp
+++ b/primitive_process/interface/subface/simple/extrude_polyline_side_face.hpp
@@ -9,8 +9,8 @@ namespace internal
 {
 
 // ==================== 前置声明：辅助函数 ====================
-paired_model_matrix compute_polyline_bottom_cap_matrix(const polyline_descriptor_t& desc);
-paired_model_matrix compute_polyline_top_cap_matrix(const polyline_descriptor_t& desc);
+paired_model_matrix compute_polyline_bottom_cap_matrix(const extrude_polyline_geometry_t& geom);
+paired_model_matrix compute_polyline_top_cap_matrix(const extrude_polyline_geometry_t& geom);
 
 // ==================== Polyline 拉伸侧面 ====================
 
@@ -77,13 +77,14 @@ struct extrude_polyline_side_face_t : subface {
     internal::paired_model_matrix get_bottom_cap_matrix() const
     {
         assert(geometry_ptr);
-        return compute_polyline_bottom_cap_matrix(geometry_ptr->descriptor.axis);
+        // 传入完整几何对象，使 calculate_tangent() 能正确处理圆弧段
+        return compute_polyline_bottom_cap_matrix(*geometry_ptr);
     }
 
     internal::paired_model_matrix get_top_cap_matrix() const
     {
         assert(geometry_ptr);
-        return compute_polyline_top_cap_matrix(geometry_ptr->descriptor.axis);
+        return compute_polyline_top_cap_matrix(*geometry_ptr);
     }
 };
 
diff --git a/primitive_process/src/subface/geometry/extrude_helixline_geometry.cpp b/primitive_process/src/subface/geometry/extrude_helixline_geometry.cpp
index 1faab2d..2952aa1 100644
--- a/primitive_process/src/subface/geometry/extrude_helixline_geometry.cpp
+++ b/primitive_process/src/subface/geometry/extrude_helixline_geometry.cpp
@@ -31,13 +31,12 @@ void extrude_helixline_geometry_t::build()
               << "original profile points: " << profile_desc.point_number << ", after fillet: " << filleted_desc.point_number
               << "\n";
 
-    
+
     const auto&           matrix_handle = this->axis_to_world.matrix();
     const Eigen::Vector3d N             = -matrix_handle.col(0);
-    const Eigen::Vector3d T             = (matrix_handle.col(2) * this->axis_geom.height
-                               + this->axis_geom.radius * this->axis_geom.total_theta * matrix_handle.col(1))
+    const Eigen::Vector3d B = (this->axis_geom.height * matrix_handle.col(1)
+                               - this->axis_geom.radius * this->axis_geom.total_theta * matrix_handle.col(2))
                                   .normalized();
-    const Eigen::Vector3d B = N.cross(T);
 
     // 构建 profile 几何数据（使用插入圆角后的 descriptor）
     aabb_t_dim<2> profile_aabb;