An integrated optimization design method for rectangular mirror was applied in this paper aiming at the requirements of high lightweight ratio and high surface figure quality of the primary mirror in off-axis three-mirror optical system for space optical remote sensor. Firstly, the topology optimization method with variable density was used to obtain the mirror topology models with maximum stiffness and maximum first-order frequency respectively, the integrated design scheme of the mirror was obtained by synthesizing the two topology models. Then, a variable expansion optimization method of the mirror ribs based on the mirror deformation value was proposed, the sensitivity analysis of the design variables was carried out, and the multi-variable size optimization of the local ribs of the mirror was optimized, Finally, the optimization result was analyzed by finite element method. The result of finite element simulation analysis shows that the surface figure of the mirror after optimization has been significantly improved. The weight of the mirror after size optimization design is only 8.65 kg, resulted in a weight reduction of about 80.9% compared to the initial solid mirror. Under the loading condition of 1 G gravity in Z direction, The PV value is 23.70 nm, with RMS is 4.54 nm. which meets the design requirements.