In order to reduce the effect of in-orbit space environment on the optical system of a laser communication terminal, improve communication quality and tracking accuracy, high-volume fraction SiC/Al that has better overall performance was used as the material of the primary mirror. Several important optimization parameters of the primary mirror structure were determined by the use of finite-element analysis. An integrated flexible support for primary mirror was proposed. The problem of stress concentration was solved because of using mismatch expansion coefficient materials for support assembly. The temperature stability of primary mirror surface shape was also improved. On the basis of this work, the weight of primary mirror and its support were reduced, a lightweight optical system was achieved. Simulation analysis shows that the PV of wavefront aberrations is λ/52 and the RMS of wavefront aberrations is λ/275, when the primary mirror under condition of gravity release. When the working environment temperature changes ±4 ℃, the PV of wavefront aberrations is λ/11 and the RMS of wavefront aberrations is λ/71. The base frequency of primary mirror and its support is 208 Hz. The lightweight rate of primary mirror is 55.3%. After integrated design, the lightweight rate of primary mirror and its support is 19.87% compared to conventional design. In summary, system overall specifications requirements can be met.