Abstract: In satellite structural design, the traditional structural optimization method based on global frequency constraints could not be directly applied to the local frequency constraints with mode switching. Therefore, an approximation method was proposed in the present work. It is assumed that the change in the overall mass matrix of the structure could be neglected during the optimization, and the modal shapes of the specified local frequency were consistent before and after optimization. Based on those two assumptions, the original optimization for local frequency constraints was transformed into an optimization for nodal displacement constraints of the local region, and that the nodal displacement constraints were determined by the ratio of the present local frequency to the expected local frequency. Both numerical and practical engineering optimization examples verified the feasibility and effectiveness of the proposed method.