Aiming at the working conditions and working requirements of the fast steering mirrors for space cameras, a lightweight scheme for fast steering mirrors was proposed. Taking the mirror with a diameter of 100 mm as the research object, the lightweight structure based on ribs was designed, and the calculation method of the equivalent thickness of standard circular mirror was proposed. Two supporting methods of the mirror, rear support with three points or rear support with a central point, were designed separately, and the results of the contrastive analysis indicate that mirror supported with a central point can avoid mutual interference between multiple support points, which is caused by inconsistent temperature deformation of the holder and the mirror. The accuracy of the mirror surface is higher, and because of its simple structure, the total mass of the wobble assembly is lighter. In order to achieve the best overall performance of the fast steering mirror structure, a multi-objective optimization of the main structural parameters was executed, the total mass of the wobble assembly and the RMS value of the mirror surface were simultaneously used as the optimization goals. The optimization results showed that the rib height and the adapter thickness contributed the most to the overall performance of the structure. The total mass of the wobble assembly under the optimized scheme is only 95.75 g, and the first-order resonant frequency of the structure is 217 Hz, the RMS of the mirror surface is 7.26 nm under the condition of -8 ℃ temperature load, which meets the design requirements while achieving a 40.4% reduction in the weight of the mirror.