The invention provides a preparation method of an RB-SiC (Reaction Bonded Silicon Carbide) substrate reflector surface modification layer, and belongs to the technical field of thin film deposition. The method comprises the following steps: I, preparing filming conditions: mounting a Si target on a twin sputtering cathode of a filming machine, performing surface cleaning treatment on the RB-SiC substrate, fixing to a filming machine workpiece clamp, vacuuming the filming machine and roasting the RB-SiC substrate; II, performing surface treatment on the surface of the Si target of the sputtering cathode: introducing Ar gas into the sputtering cathode, and adjusting medium frequency power supply power to perform sputtering pre-treatment on the surface of the Si target; III, preparing the Si modification layer: increasing the medium frequency power supply power and Ar ventilation volume, and depositing the Si modification layer on the RB-SiC substrate by using a medium frequency magnetron sputtering technology. The preparation process is greatly simplified, the compactness of the film layer is high, the film is uniform, the polishing characteristic is obviously improved, and the optical quality of the surface of the polished substrate is greatly improved.