Off-axis Three-mirror system (TMA) is widely used in space remote sensors with high resolution and wide field of view, in which rectangular reflective mirror is regarded as the core component, for its performance directly influences imaging quality. Based on traditional light-weighted structure for reflective mirror, bring forward a plan with variable rib thickness design in order to improve local surface quality. Take both the quality of reflective surface and the mass of mirror as objectives, use NCGA algorithm to solve the multi-objective optimization problem and ascertain the best combination of structure parameters. Modify the optimized design according to manufacturing constraints. Under the load condition of gravity in z-direction, the RMS value of final design is 1.278 nm, with the PV value of 5.642 nm, which is respectively 34.4% and 32.4% smaller than the original design (RMS=1.947 nm, PV=8.349 nm ). Meanwhile, the mirror is lighter, satisfying the demand of high surface quality and light structure. This suggests that the light-weighted plan and the method of determining parameters proposed in this paper are effective for the light-weighted design of rectangular reflective mirror used in TMA system, and have reference value for future development of related elements.