An optical supporting structure is studied to improve its structure stiffness and reduce the structure mass to meet the requirement of space camera. A formula to calculate fundamental frequency is proposed using Rayleigh method, and the influences of the angles between trusses on fundamental frequency are analyzed. Then, the optimization method and process are studied. Based on the analysis above, an optical supporting structure with high stiffness for a pre-research space camera is designed. The analysis results indicate that the mass of designed optical supporting structure has been reduced by 22 kg and its fundamental frequency is 146 Hz,which shows that above analysis has a certain instructional significance for the design of optical supporting structures in high-resolution off-axis three-mirror reflective space cameras.