Abstract: In terms of the strict design requirements on the stability and mass of equipment for the airborne platform, an opto-mechanical structure design method of a lightweight primary mirror assembly is proposed. In this study, based on the analysis of the optical index of the laser beam expander system, the mechanical design index of the primary mirror is obtained. Combined with the environment requirements of the airborne platform, the support scheme of the mirror and lightweight design are studied. The engineering analysis and test of the primary mirror assembly are carried out. The results show that the safety factor of the bonding strength between internal components reaches 2.7. The RMS values of the mirror surface shape error are better than λ/50 (λ=632.8 nm) in the horizontal state of optical axis, the vertical state of optical or the uniform 5℃ temperature rise. In the random vibration environment, the amplification factor of the acceleration response RMS value of the mirror sensitive point is 6.29. The surface shape error test is carried out, and the RMS value of the surface error is 0.018 λ. These results demonstrate that the mirror assembly design method is reasonable, meets the design requirements. It has important guiding significance for the similar structural design.