In order to achieve high quality optical aspheric silicon carbide (SiC) mirror, the SiC surface must be modified primarily. Firstly, the modification technology of ion beam assisted deposition (IBAD) for depositing silicon (Si) on SiC mirror surface is introduced. Then, some characteristics, such as mechanical performance, optical fabrication capability, surface roughness and reflectivity, of Si modification layer on SiC surface are studied. The results of experiments show that the mechanical performance and the optical fabrication capability of Si modification layer on SiC surface are excellent. After polishing, the surface roughness and reflectivity of the Si modification layer on SiC are 0.85 nm \[root mean square (RMS)\] and 98.5%, respectively. Finally, an example of Si-modified SiC off-axis aspheric mirror (Ф600 mm) is polished by computer-controlled optical surfacing technology. The testing result indicates that the surface accuracy of the mirror is 0.018λ （λ=0.6328 μm） RMS. It can meet the technical requirements of high quality space optical aspheric mirror.