a phi 600mm sic primary mirror subsystem of a space-borne ritchey-chretien telescope was designed. the open-back primary mirror was made of pressure-less sintering silicon carbide ( sic), light-weighted at a ratio of approximately 70%. minimizing the optical surface astigmatism was critical for the mirror, the astigmatism is caused mainly by gravity effects, temperature variation and the bonding process. three invar flexure bipods were fixed on the baseplate of the telescope at first, and the posture of the primary mirror was adjusted precisely for 0.2mm gap to the bipods. 3m 2216 b/a grey adhesive was then injected into the gap between the mirror and bipod flexure, the curing process was last 72 hours in the room temperature. so the mirror was affected only by curing stress of the adhesive during the assembly process. structural strength and dynamic stiffness of the mirror subsystem in the thermal-structural coupling state were analyzed with finite element method. analyzed results show that the optical surface distortion is less than 1/50 lambda at 632.8nm rms with three points support and less than 1/200 lambda rms with 2 degrees c temperature variation because of the flexure support and compatible support and mirror material, the optical performance test with interferometer show that the optical surface distortion caused by the curing stress of the adhesive is less than 1/50 lambda rms, the overall optical surface of the primary mirror is less than 1/30 lambda rms, which met the critical requirements for the primary mirror of the telescope.