Introduces the basic principle of Geometric Modeling for 3D model, and three kinds of usual methods to calculate the vertex normal line on its surface are mentioned. These usual methods make most of the 3D model with smooth surface looks well when they were rendering. But a model with obviously sharp corner and edge would not be rendered correctly. To solve this problem, the Hard Edge Splitting method is bringing out, by calculating to find out the sharing edge with the angles bigger than a threshold of two adjacent triangles. In each triangle, calculate the normal of these two vertexes on the edge line. It makes the lighting calculation for surfaces at each side of the edge correctly. The geometrical features of the model can be shown in truth.