Recently, Reaction Bonded–SiC (RB-SiC) has become the most promising space mirror material due to its superior mechanical, thermal and physical properties. But surface roughness of polished RB-SiC is limited by the Si/SiC two phase structure of the material. To solve this problem, a kind of surface coating technique was introduced to obtain ultra-smooth optical surface. Firstly, according to the application environment of space mirrors, proper coating materials were analyzed generally. Secondly, SiC deposited by new Ion Assisted Depositing (IAD) technique was chosen to be the coating material and properties of IAD-SiC film were tested. Finally, a series of polishing experiments on 6±0.5μm thick IAD-SiC layers were accomplished. Results show that surface roughness of the finished coating achieves less than 0.5nm RMS and the final surface figure is 0.033λ RMS (λ=632.8nm). The technique shows an access to process ultra-smooth and low scatter RB-SiC mirrors.