The invention discloses a manufacturing process of a SiC optical reflector based on 3D printing and diamond cutting process. The manufacturing process comprises the steps (a) the back-side lightweighting structure of the SiC optical reflector is analyzed and obtained, and a 3D model is constructed; (b) layered processing is conducted on the 3D printer, and SiC ceramic powder, deionized water, dispersant, antifoaming agent and curing agent can be made into ceramic slurry according to proportion, and a cross section thin layer can be printed and sintered layer by layer through the 3D printer, and a SiC optical reflector body can be obtained, and a light-weighting structure at the back of the body can be directly printed and molded; (c) the diamond ductility cutting process is adopted to conduct ultra-precision cutting process on the right side of the optical reflector body, and the processing surface with high precision can be obtained; (d) the mirror surface is processed through classical polishing, and the SiC optical reflector with high precision can be finally obtained. The invention is advantageous in that preparing process can be shortened, preparing time and materials can be saved, and the precision of the SiC optical reflector can be increased.