In order to satisfy the strict requirement of surface-shape and realize the ultra-lightweight design of the space mirror, the topology method was introduced into the design of a Ф500 mm round space mirror. According to the density method, the SIMP model was established. In the condition of axial-oriented gravity, taking general flexibility as design constraint, minimum volume as design objective, after iterations, better structure was gotten, whose RMS was less than 5nm and lightweight ratio reached 75.83%. In the same weight, the conventional triangular lightweight structure had the RMS of 8.17nm and lightweight ratio of 67.39%. The surface-shape contrast between topology structure and conventional structure was made in the condition of radial-oriented gravity. The computational result meets the requirements of design. The method of topology optimization is better than the conventional method in both the surface-shape and lightweight ratio.