In order to improve the optical quality and lightweight ratio of a mirror, optomechanical analysis and optimization for the primary mirror of a space camera are carried out. The mirror surface performance under multiple load conditions is considered, including static gravity and uniform temperature rise load, as well as nonuniform temperature distribution loads. Sensitivities of the surface performance to the size parameters of the mirror are analyzed. The numerical simulations show that the size parameters obviously affect the performance and which parameters are key. The computed surface performance is analyzed with the help of ray-tracing software, and the contribution of size parameters to system optical performance is further evaluated. A size parameter optimization model is established to optimize the optical quality of the mirror. Finally, engineering analysis and dynamic tests are carried out. The results show that mechanical properties of the mirror are excellent, and the relative error of finite element analysis and dynamic test is within 10%. This verifies the accuracy of the model and the effectiveness of the optimization method.