In order to meet the requirements of the surface error RMS value and stiffness of GEO(Geostationary Earth Orbit) laser communication primary mirror subassembly, the subassembly structure was optimized. Firstly, the optimal design of 9 group parameter combination of primary mirror was conducted by orthogonal optimization method, instead of total 81 group combination, avoiding repeating parameter optimization and heightening the efficiency of optimal design, in which the surface error RMS value on 1 g gravity and the one on 2 ℃ radial temperature difference were satisfied with λ/50(λ=632.8 nm) surface error demand, the RMS value under 5 ℃ uniform temperature rise can be improved. Secondly, the flexure support was optimized. The simulation result showed that the first order fundamental frequencies is 213 Hz, superior to 200 Hz fundamental frequency, the maximum surface error RMS values of 1 g gravity on three directions, 5 ℃ uniform temperature rise and 2 ℃ radial temperature difference is 10.78 nm, lower than surface error demand. The test demonstrated that the RMS value under 5 ℃ uniform temperature rise is 7.27 nm, less than design requirement. The optimal design provided technical support to process and alignment of primary mirror subassembly.