The electro-optic system is an important component of loitering munition. With the rapid development of loitering munition in recent years, the electro-optic systems of loitering munition are expected to be more compact, light, and with more excellent performance. Therefore, advanced design methods are urgently needed to solve the above problems in the electro-optic system design. In order to meet the multiple requirements of loitering munition, an effective design method for key structures of electro-optic systems based on topology optimization was proposed.The advanced topology optimization method was employed to raise the design standard of electro-optic systems. Specifically, the stiffness under acceleration conditions in X,Y, Z directions and the first modal frequency were all considered as the optimization design constraints, and mass minimization of the design structure of electro-optic systems of loitering munition was taken as the optimization objective function in the proposed topology optimization method. The variable density method was employed to establish the topology optimization model. The optimized topology layout of the electro-optic systems structure can be obtained by solving the topology optimization model with the help of Altair HyperWorks software, then it would be used to reconstruct the geometric models based on production technology with the software of Unigraphics NX. In the next step, the reconstruction model would be analyzed to confirm if the optimized design could meet all the requirements. As the key structure of electro-optic systems of the loitering munition, the optical bench was analyzed and optimized to improve the design performance using the proposed method. In this typical example, the topology optimized result of optical bench was obtained (Fig.5) and reconstructed (Fig.7). Then the modal analysis (Fig.8) and overload analysis (Fig.9) were executed to verify the performance of optimized reconstructed optical bench. The result showed that the mass of optical bench of electro-optic systems was reduced by 22.4% with stiffness under acceleration conditions in X, Y, Z directions and the first modal frequency maintaining equivalent performances (Tab.1). In the completed example, the optimization method has greatly improved the lightweight level of electro-optic systems of loitering munition, and it offers great help in meeting the requirements of loitering munition system. In especial, the optimized design of optical bench has been produced and successfully applied to the electro-optic systems. Obviously, the proposed method can also be extended to the design of other parts to improve the overall design level. In this study, an effective design method for key structures of electro-optic systems based on topology optimization is proposed. The mass, stiffness and the first modal frequency of the key structure are all considered in the optimization design method to ensure design effectiveness. As a typical example, the optical bench was analyzed, optimized, reconstructed and checked successively during the whole design flow and the optimized design of optical bench has also been produced and verified in the physical test. Compared to experiential design, the topology optimized optical bench has a significant advantage in weight and stiffness. The result of the example showed that the proposed topology optimization method can effectively benefit the lightweight design of the electro-optic systems of loitering munition. Therefore, the topology optimization method for key structures of electro-optic systems of loitering munition has great appliance and good popularization value.