A improved technological process to the surface modification of a RB-SiC substrate mirror was proposed according to the material special properties of the large scale and complicated light weighted RB-SiC materials for space projects. Using a high energy Kaufman ion source for the Ion Beam Aided Deposition(IBAD),the substrate was pre-carbonized and coated by a carbon buffer layer. Then,a Si layer was coated upon the buffer layer for surface modification,and the properties of the sample were tested. In comparison to the process simply using a Hall ion source for the IBAD,the coating has a more compact and uniform structure and a better property in polishing. After surface modification,the surface roughness(rms) of the substrate is reduced to 0.635 nm which is the same as that of the S-SiC substrate,also the surface reflectance is obviously enhanced to that of the fine polished zerodur glass. The conclusion can be drawn that this technological process of surface modification for the RB-SiC mirror is quite reasonable and effective.