In order to obtain a lightweight primary mirror that meets the engineering requirements, the large-diameter SiC primary mirror of an optical measurement system is taken as the optimization target, and the support structure of the primary mirror is firstly determined, and then the structural dimensions of the primary mirror are optimized, and its thickness is optimized by grouping the reinforcement bars of the primary mirror, while the thickness of the front and rear panels as well as the total thickness of the primary mirror are optimized. Subsequently, the structure of the primary mirror is optimized by proposing a structural form in which triangular holes are used in conjunction with fan-shaped holes, and the face shape of the primary mirror is greatly improved. The mass of the final primary mirror is reduced by 16.7 kg compared with the initial primary mirror, the diameter-to-thickness ratio is increased from 11.4 to 12.8, and the static and dynamic characteristics meet the requirements. The influence of axial and radial temperature difference on the optical system is analyzed, and the modulation transfer function of the optical system is used as an evaluation criterion to determine the range in which the axial and radial temperature difference of the primary mirror should be controlled. Finally, the surface shape accuracy of the primary mirror is examined by a four-dimensional interferometer to verify the correctness of the finite element analysis.