An innovative design scheme based on semi-closed backplane structure was proposed to meet the requirements of lightweight and stiffness improvement of SiC planar mirror in two-dimensional pointing mechanism of space camera. The semi-closed topological configuration on the back was constructed to replace the traditional open structure, and the stiffness was improved by adjusting the size of the process hole. The finite element model was established and the three design schemes were compared. The simulation results showed that the mass of the second scheme was reduced from 12.8 kg to 10.7 kg, the first natural frequency was increased from 730 Hz to 1528 Hz, and the ratio of stiffness to mass was increased by 1.5 times. The performance index was significantly better than that of the traditional open structure (scheme 1) and the large aperture scheme (scheme 3). The optimized pointing mirror had passed the sinusoidal vibration test, and the RMS value of the surface shape accuracy was doubled from 0.073 λ to 0.036 λ (λ = 632.8 nm). The semi-closed backplane design proposed in this study breaks through the stiffness limitation of traditional lightweight structures, and provides a new method for the design of other space mirrors.