The primary mirror is an important optical component of space camera. Its performance related to the optical image quality, and the weight directly affects the whole camera weight. The traditional design of primary mirror relies on much experience, lacking of precise theory, and many design parameters obtained by empirical formulas, thus the performance of the result is unstable. For this study, a primary mirror made of SiC with the diameter of 700mm was conceptual designed to get the optimized structure. Then sensitivity analysis was carried out to determine the optimum thickness of the back muscles. Finally, the optimum primary mirror fully satisfied the required was completed, with outstanding mechanical performance and light weight. A comparison between the optimum primary and traditional primary was performed and the results showed that the optimum primary has higher lightweight ratio increased by 5%, higher modal frequency increased by 81Hz.The maximum deformation under gravity reduced by 48nm, PV of the mirror surface reduced by 8.1nm and RMS reduced by 3.1nm. All the results indicated that the optimization method in the paper is reasonable and effective, which gives a reference to the primary mirror design in the future.