As the aerospace technology develops rapidly, more requirements on the aerospace optic system are needed. With excellent physical properties, SiC becomes a very promising material for speculums. The mechanism of polishing on SiC mirror surface is studied, the experimental method of SiC polishing is introduced, and the polishing process is analyzed quantatively. The key factors affecting the surface quality of SiC mirror, such as tools, abrasives, load, rotational speed and slurry pH are studied, chosen and optimized by plenty of experiments and theoretical analysis, and better results are obtained.