In order to improve the speed of CNC milling simulation, a method for fast rendering of differences between entity sets was proposed. By applying space partition of the entities, the dimension of the problem was reduced and fast rendering could be achieved. The number of sweeping volumes of milling tools in the rendering sequence and the repeated drawing of primitives could be reduced by subdividing the viewing frustum. At the same time, the data of the sub-window was independent in the drawing process, facilitating the parallel generation of images. Several experiments demonstrated that the proposed algorithm can achieve the real-time rendering of CNC milling process simulation, and provided the processing simulation results of several industrial parts by analyzing and experimentally verifying the parameters of the algorithm.