To actualize precise polishing of silicon modification layer on silicon carbide surface, and achieve high-quality optical surface, a technology of magnetic-medium assistant polishing is studied. The special magnetic polishing tool for polishing silicon modification layer on silicon carbide surface is designed, and the material removal function of magnetic polishing tool is studied. On the basis of the material removal function, the dwell time algorithm of computer-controlled magnetic-medium assistant polishing is researched. A Si-modified on silicon carbide surface experimental sample is polished by magnetic-medium assistant polishing technology. Initial figure error and roughness of the sample are 0.049λ（λ=0.6328 μm) (root-mean-square) and 2 nm. After one polishing iteration, the figure error and roughness of the sample are 0.015λ（λ=0.6328 μm) and 0.64 nm, respectively. Experimental results indicate the dwell time algorithm based on matrix algebra is effective, and magnetic-medium assistant polishing is appropriate for polishing silicon modification layer on silicon carbide surface.