Abstract: In order to satisfy the strict requirements of the surface-shapes and lightweight ratios for space mirrors, the topological method was introduced to design the reflect mirror with a Ф 700 mm. According to the density method, the Solid Isotropic Microstructure with Penalization(SIMP) model was established. In the condition of axial-oriented gravity, by taking the general flexibility as a design constraint, and the minimum volume as a design objective, a better structure with a RMS of 8.89 nm and a lightweight ratio of 82% was obtained after iterations. In the same weight, the conventional triangular lightweight structure shows its RMS in 11.75 nm and lightweight ratio in 65%. In the condition of radial-oriented gravity, the surface-shape of the topological structure also meets the requirements of the design. In the considerations of the surface-shape and lightweight ratio, the computational result shows that the topological optimization method is better than the conventional method.