The SiC material can't meet the application requirement of high quality space-used optical system because the considerable surface scattering on the SiC mirror after polish. So the surface modification must be taken on in order to obtain high quality optical surface on the SiC mirror before use. The two prevailing international methods for surface modification are to use of Si coating and SiC coating. Surface modifications are made by the two means using e-beam evaporation with ion assisted. Test results show that the Si coating is cubic phase. Surface roughness(rms) of the SiC substrate is reduced to 0.620 nm and scattering coefficient is reduced to 1.52% after Si coating modification. The SiC coating is amorphous. Surface roughness(rms) of the SiC substrate is reduced to 0.743 nm and scattering coefficient is reduced to 2.79% after SiC coating modification. The modification coatings both have a good temperature stability and a high adhesion to the SiC substrate. Evaluated by the reliability the method of using Si coating is much more compatible for the engineering application interiorly. After the surface modification by means of using Si coating, the wastage of surface scattering is reduced greatly and the quality of optical surface is improved obviously. The surface reflectance with Ag coating is obviously enhanced to that of fine polished zerodur glass, thus it can meet the application requirement of high quality space-used optical system.