Magnetorheological finishing (MRF) process is adopted to overcome the drawback of traditional polishing method in final finishing of SiC after silicon modified and removing the debris of optical surface in final finishing with high efficiency and fast convergence. The requirements of magnetorheological (MR) polishing fluid are proposed according to the actual polishing situation. The rheological characteristic and dispersion stability are tested, which verify that the prepared MR polishing fluid possesses good performance. A silicon modified reactbonded coaxial asphere mirror with diameter of 130 mm (effective sub-aperture is 120 mm) is polished with two iteration in three hours approximately. The root-mean-square (RMS) of the surface accuracy is improved from 0.051λ (λ=632.8 nm) to 0.012λ rapidly and the surface roughness Ra reaches 0.618 nm. The results proves that the prepared MR polishing fluid can satisfy the polishing requirement of modification layer on silicon carbide and MRF possesses outstanding advantages in final finishing of silicon modified mirror.