In order to achieve a high degree of light weight, a sector-shaped SiC light-weight primary mirror of a 1000 mm photoelectric theodolite is taken as the research object and optimized design. By optimizing the thickness of the ribs on the back of the main mirror, the thickness of the semi-closed back panel and the total thickness of the main mirror, the volume and quality of the main mirror are reduced. After the three-dimensional model is established, the finite element model is established using the finite element software Abaqus, and the deformation analysis of the primary mirror is performed after the finite element simulation results are obtained. The Zernike polynomial is used to fit the deformation data of the primary mirror, and the root mean square (RMS) value of the surface error of the primary mirror is obtained. The simulation results show that the mass of the optimized main mirror is 62.78 kg, which is 30% lower than the initial mass (89.36 kg), and the diameter-to-thickness ratio of the main mirror is increased from 8.58 to 11.44 when the design requirements of the main mirror surface accuracy are met. When the optical axis is horizontal, a four-dimensional interferometer is used to detect the surface shape of the fan-shaped lightweight main mirror, and the detection result of the RMS value of the main mirror surface error is 18.22 nm.