SiC ceramics，as a new generation of mirror materials with excellent comprehensive properties，have been widely used in space camera mirrors. However，in the late stage of SiC optical processing，polishing continues after surface modification to reduce the surface roughness and improve the surface quality，will directly affect the process accuracy and cycle of SiC mirror. Therefore，it is necessary to improve and optimize its optical processing technology. In this paper，KMnO₄ additive is introduced to analyze the chemical reaction mechanism involved in the reaction. Strong oxidizing reacts with the surface of the SiC sample，which reduces the grinding amount of diamond powder to SiC mirror and achieves the purpose of reducing the surface roughness. For the series of diamond powder polishing the 200 mm diameter atmospheric pressure sintered SiC sample，the surface roughness was correspondingly reduced with KMnO₄ additive. It was interesting that W7 abrasive with KMnO₄ additive polishing SiC surface reached the similar roughness effect of W3 abrasive. The obtained mirror roughness S_q （4D，10 times magnification） was 1.628 nm after being polished with W0.1 diamond powder and KMnO₄ additive. The result was better than the polished super smooth surface quality of modified SiC. This realized the efficient and high-precision processing of SiC mirror with the introduction of KMnO₄ additive.