Neckline, wristband and pantalets are key parts of 3D garment models. The extraction and editing on these boundaries play an important role in 3D garment fitting application. This paper firstly extracts boundary points, uses the non-uniform B-spine curve of degree three to interpolate them and calculates the length of each boundary. Then we add new spatial points by constructing the parallel tangent plane based on boundary points and create the extending boundary surface by triangulating these points. Finally, it splits the boundary part of the garment by the cutting plane to form a new boundary style of the garment. The experimental results show that our method is effective and applicable for the local adjusting and editing of 3D garment mesh models.