In order to eliminate the surface microdefect after the direct polishing of RB-SiC substrate, reduce the surface roughness and increase the surface quality, Si was selected as the modified material based on the features of large aperture SiC, where a 2 m-level RB-SiC substrate was modificated by using the magnetron sputtering technology. The silicon film was deposited by a developed Φ3.2 m magnetron sputtering coating machine, and the SiC substrate was polished and modificated based on the computer control optical molding method. The result indicates that the thickness of modified level reaches 15 μm; the thickness uniformity of the film level is better than ±2.5% within the diameter of 2.04 m; the surface roughness decreases from 5.64 nm (RMS) to 0.78 nm. Therefore a the magnetron sputtering technology can be used in the surface modification of the large aperture RB-SiC, and the performance of the modificated RB-SiC substrate can meet the requirements of high-quality optical systems.