As pin holes of sintered silicon carbide formed in manufacturing process cause serious surface scattering of mirrors, this paper proposes a surface modification technique to decrease the surface scattering of sintered silicon carbide mirror. A silicon layer was deposited by plasma ion assisted deposition technique on the mirror as a surface modification coating to eliminate the defects on the mirror surface and to decrease the surface scattering effectively. A scanning electron microscope was applied to characterize of the morphology of the sintered silicon carbide sample, and the surface scattering origin was systematically analyzed. A total integrated scattering instrument was established. Then the total integrated scatterings of the sintered silicon carbide samples before and after surface modifications and a fine polished K9 glass sample were measured. The results indicate that the total integrated scatterings of the sintered silicon carbide sample before and after surface modifications are 3.92% and 1.42%, respectively, and that of the K9 glass sample is 1.36%. The surface root mean square roughnesses of the sintered silicon carbide sample after surface modification polishing and the fine polished K9 glass sample are 1.63 nm and 1.04 nm respectively. These results demonstrate that the defect of the sintered silicon carbide is successfully eliminated after surface modification, the surface scattering is greatly decreased, furthermore the surface optical performance is quite close to that of the K9 glass.