According to the design targets of a mirror which is made with the material SiC in a certain space optical system, analyze and contrast the calorific capability (which is figured by the deformation caused by the different temperature) and the mechanical capability (which is figured by the deformation caused by the gravity) of many different light-weighted structures (such as triangle, square, hexagon and various sectors). Then, decide to use one of the sector light-weighted structures after contrasting and balancing the light-weighted percent, rigidity ratio (which is the ratio of the elastic model-E and the density of the material) and the calorific stability. With the sector light-weighted structure, the light-weighted ratio reaches 75 percent, and the deformation brought by the gravity and the calorific deformation are both enough small for use. The RMS of aberration-deformation of the mirror surface with surrounding support is 0.010 1 μm which is about λ/60 (λ=0.632 8 μm). The aberration-deformation does not contain the rigid displacement of the mirror surface which is removed by using the Zernike polynomial to fit the deformation of the mirror surface calculated by the MSC software.