Aiming at the requirements of lightweight and stiffness improvement of silicon carbide (SiC) plane mirror in the two-dimensional pointing mechanism of space camera, an innovative design scheme based on semi-closed backplate structure was proposed. By constructing a semi-closed topological configuration on the back and replacing the traditional open structure with it, the stiffness was improved by combining the control of the process hole size. The finite element model of the pointing mirror was established, and three design schemes were compared. The simulation results show that, under the premise of ensuring the surface shape accuracy index, the mass of scheme 2 was reduced from 12.8 kg to 10.7 kg (a decrease of 16.4%), the first-order natural frequency was increased from 730 Hz to 1528 Hz (an increase of 109%), and the stiffness-to-mass ratio 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 passed the sinusoidal vibration test, and the root mean square (RMS) value of the surface shape accuracy was doubled from 0.073λ to 0.036λ (λ=632.8 nm). The semi-closed backplane design proposed in this study breaks through the stiffness limitations of traditional lightweight structures and provides a new approach for the design of other space mirrors.