This study proposes a design method combining variable density topology optimization and multi-objective integration optimization with minimum size constraints for the overall design of multispectral cameras, adhering to the design concept of integration, miniaturization, and ultra-lightweight. This approach is used for spatial separation, and the optimized design of an axis microcrystalline mirror and a back support structure (heat dissipation mandrel, flexible joint, and backplate). The mirror has an aperture of 218 mm×166 mm, mass of 0.917 kg, and weight reduction rate of 71.3%. In addition, the processed and assembled reflector components are subjected to engineering environmental tests and surface interference detection. Test results show that the root mean square value of the mirror surface before and after the environmental test is greater than 1/50λ, detection wavelength λ=632.8 nm, and fundamental frequency is 397.8 Hz, meeting the design requirements of an optical system and optical satellite platform. Load requirements validate the feasibility and accuracy of the proposed method.