In order to improve imaging quality of SiC reflective mirror,an improved scheme of traditional lightweight mirror and the corresponding optimization method were proposed.The scheme of variable rib thickness design for lightweight mirror was brought forward.Lightweight ribs were divided into different groups based on geometry position and force transferring route respectively,both schemes with grouping performed better than traditional structure in imaging quality,indicating that lightweight ribs with different topology relation played different roles in enhancing the stiffness of lightweight mirror.The ribs on the back of mirror were trimmed in order to improve local distortion in areas away from supporting holes.The integrated optimization model based on improved structure was built,using Multi-island Generic Algorithm to find the best combination of all the structure parameters.The simulation results show that after optimization,the grouping scheme based on force transferring route overmatches traditional design,with surface quality improved by 33.2%,and mass decreased by 11.7%.