Abstract: In order to obtain the high surface quality of physical vapor deposition (PVD) modified RB-SiC mirror and solve the problems of the surface defects in the practical processing, we conducted studies on the forming mechanism and the processing method of defects. According to Preston hypothesis, it was deduced that higher impact on the large rimes in PVD layer could be the main reason which may cause surface defects. The experiment proved that the defects would appear if the polishing speed or the polishing presure was too high. In order to reduce and avoid surface defects effectively, it was reasonable to control the relative polishing speed and polishing pressure, in the meanwhile, the polishing efficiency could be maintained. According to the analysis above, the surface defects could be renovated after 0.7 m～1 m of coating material was removed with the proper polishing speed and pressure, while new defects did not appeare any more.