In order to improve the design efficiency of the space camera reflective mirror subassembly, a parameter optimization design method was proposed taking a rectangular reflective mirror subassembly as an example. Focusing on the research of the mirror and the flexible component, the optimization design process of its structural parameters was explained in detail. Finally, the optimized subassembly was simulated and analyzed. The results show that the RMS value reaches respectively 1.60 nm when gravity load is applied in the directions of Z axes, and the RMS value is 6.70 nm when the mirror subassembly is under the load condition of uniform temperature rise of 5 ℃, which are far less than the requirement of RMS≤λ/50 (λ=632.8 nm). In addition, the mass is 2.58 kg and the fundamental frequency is 274 Hz, which meets the dynamic stiffness requirement. To sum up, the parameter optimization design of the mirror subassembly structure can better meet the index, and the structure parameter optimization iteration by the computer can significantly improve the design efficiency.