A Φ450 mm primary mirror subsystem of a space-based astronomy telescope was designed with mass, optical surface distortion and reflectivity requirement. The open-back primary mirror was made of pressure-less sintering silicon carbide, light-weighted at a ratio of approximately 70%. Three side supporting invar flexure bipods were designed to minimize the assembling stress and the thermal stress. The high reflection was obtained from the optical surface cementite. The mirror weighted 7 kg and the reflectivity was 98% after optical polishing. The mirror subsystem was precisely assembled under the strict technical condition. The optical test with interferometer show that the optical surface distortion is less than 0.02λ RMS, which meet the critical optical requirements for the primary mirror of the space-based astronomy telescope.