Reaction bonded silicon carbide (RB-SiC) is a kind of mirror substrate material with good performance, however, it′s difficult to obtain fine surface without special treatment for the intrinsic defects. Xray diffraction (XRD) is used to measure the crystal structure of the RB-SiC substrate, polycrystalline silicon carbide and polycrystalline silicon are the main composition. The residual pin holes and steps on the surface observed by scanning electron microscope (SEM) and atomic force microscope (AFM) cause the scattering and decrease the optical performance directly. A silicon surface modification coating is deposited on the RB-SiC surface by the plasma ion assisted deposition (PIAD), and the defects are eliminated, a fine optical surface is achieved after polishing the silicon surface modification coating. Total integrated scattering of bare RB-SiC and the surface after surface modification is measured by a self-made scatterometer, the results are 9.37% and 1.84% , respectively, the value after modification is only 1/5 compared to that of the bare RB-SiC. Great optical performance improvement of the RB-SiC mirror is achieved, and it′s almost close to the performance of the fine polished K9 glass.