As the aperture for a monolithic mirror was limited in 1.2 m by traditional technologies, a new reaction-formed joint technology for RB-SiC was proposed. With the proprosed technology, the SiC mirror was joined in the green body process, and the green body densification and the joint of mirror were finished in the same sintering process. A 230 mm diameter RB-SiC mirror was fabricated by this technology. After milling and polishing finely with the FSJG-2 facility, the figure error of the mirror surface is less than λ/50(λ=632.8 nm) . The mirror was tested in the temperature range of (20±3) ℃, and the tested results show that the change of the figure error of the mirror surface is less than λ/300, and the mirror surface figure is not changed obviously after the thermal cycle test. Moreover, the roughness of the surface near the joint line is 3.3 nm, and its microstructure is similar to that of the RB-SiC ceramic. The thermal property of welding line is matching with that of RB-SiC ceramic. Obtained results demonstrate that the reaction-formed joint technology for the RB-SiC mirror satisfies the need of the large aperture mirror used in space optics.