To address the lack of suitable methods for automatic 2D line drawing generation from grotto wall mesh models, as well as the difficulty of existing methods in balancing structural representation and detail preservation, this paper proposes a line drawing generation method based on depth map and normal map fusion. The method first orthographically projects the 3D model into a depth map and a surface normal map, then constructs an initial line drawing pipeline based on projected edge fusion. A layered optimization strategy is further introduced to improve detail representation and result stability. Experiments on the mesh model of the north wall of Cave 18 at the Yungang Grottoes show that the projected edge fusion method is more suitable for overall structural representation, while the layered optimization method performs better in preserving weak structures and fine details. The proposed method effectively improves the quality of automatic 2D line drawing generation for grotto wall surfaces.