Fabrication techniques of lightweight silicon carbide (SiC) mirror blank were researched and the key problems in the fabricating process were discussed. An advanced lost mould method was proposed to fabricate the excellent lightweight structure with a partly open-back. In consideration of the difficulty on fabricating SiC ceramics with larger sizes and complex shapes , the gelcasting for a SiC green body was studied and the ingredient of the SiC slurry with high solid phase and low viscosity,control of slurry time and the drying processing for the wed bland in the gelcasting were discussed. Thermogravimetic and Differential Thermal Analysis TG/DTA curves for the green body were analyzed, then the best sintering process was established. By the proposed technologies, RB-SiC mirror blanks with different shapes and lightweight constructions were prepared and the largest one is 1 100 mm×820 mm.