A lightweight and optimization design method for a medium aperture rectangular mirror supported in centre was proposed to meet the requirement of designing lighter and better satellite cameras. By choosing the method of rear support in centre by single point, the mass of both mirror and subassembly was decreased and the design of the support structure was simplified. By using multi-objective optimization design, the surface figure accuracy under the load case of gravity in Z-direction was improved. A flexible support structure dedicated to the mirror supported in centre was designed to overcome its shortcomings of low stiffness and low dynamic reliability. The integrated performance of the mirror was simulated and compared with that of the mirror mounted via rear three points. It shows that the mirror supported in centre has a lighter mass of 3.36 kg and the lightweight ratio is 87% compared with the solid mirror, and that the mass of the mirror subassembly is decreased 24% of the one supported by three points. The surface figure accuracy RMS of the mirror reaches respectively 2.2, 2.1 and 7.5 nm when gravity load is applied in the directions of X, Y and Z axes, which is better than that of the one supported by three points. Furthermore, the RMS is 2.8 nm when the mirror subassembly is under the load condition of uniform temperature rise of 4 ℃, which is far less than the requirement of RMS≤12 nm. Otherwise, the first order natural frequency of the mirror subassembly is 135 Hz, and the maximum rigid body displacement is 3.96 ?滋m. The proposed design method not only reduces the mass of the mirror with its support structure extremely, but also ensures the surface figure accuracy of the mirror and the dynamic and static rigid of the mirror subassembly requirement, provides a new approach to lightweight and optimization design for the same type space mirrors.