Abstract: In order to reduce the volume and weight of space optical load, and ensure that the system has good temperature adaptability and dynamic stiffness, a lightweight coaxial optical mechanical system with Φ400mm diameter and full SiC material is designed. Using the parameter optimization method, the system fundamental frequency and surface shape accuracy RMS value as the optimization objective, and the weight of the whole machine as the constraint, the axial thickness of the support and other related parameters as the design variables, the three-point cantilevering ultra-light coaxial optical mechanical structure is obtained. The rationality of the cantilever ultra-light coaxial optical mechanical structure is verified by simulation analysis, vibration test and surface shape test. The results show that the weight of the optical mechanical system is only 14kg, the system wave aberration is 0.05λ（λ=632.8nm） in the temperature range of 20±4℃, the fundamental frequency of the system is 142.35Hz, the simulation analysis results is 138.04Hz, the error is 3% with the simulation analysis, which can ensure the stability of the whole machine structure.