1. A 3D model rendering method comprising: drawing a sketch of a 3D model; calculating, in a diffuse reflection illumination situation simulated by hardware, dot product operation results of a light vector and a normal line vector of each vertex on a surface of a 3D model, the light vector corresponding to the diffuse reflection illumination, and converting the dot product operation results of each vertex to corresponding UV coordinate values; drawing, according to a preset correspondence between UV coordinate values and a color value of a 3D model basic texture after receiving light, a gradient map having a color value corresponding to the UV coordinate values of the each vertex; and covering the surface of the 3D model with the gradient map; wherein the drawing a sketch of a 3D model comprises: enlarging, according to a preset stroke line adjusting manner, the 3D model along a normal line direction of each vertex, cropping a side face of a side that runs outward along a normal line of each triangular facet on the enlarged 3D model, and coloring the cropped 3D model by using a preset stroke color.