Abstract: The transmission frame in launch vehicles is a key structure to transfer the thrust load between the rocket body and engine. The structural lightweight design can not only ensure the thrust-weight ratio of engine, and improve the stability of rocket, also provide reference values for the reusable launch vehicles in the future research. Under the moving morphable component (MMC)-based framework, this paper proposes an approach for solving lightweight design problem of transmission frame structures. In this method, structural topology can be described by using a set of morphable components with explicit geometric information, which renders the optimized layout in terms of a small number of design variables. By analyzing the characteristics and requirements of transmission frame structures, structural stiffness maximum under volume constraint is chose as the objective function, and the corresponding problem formulation based on the MMC explicit topology optimization method can be formulated. Thus, an optimization platform is established to achieve the lightweight design of transmission frame in practical engineering. Under two different load cases (i.e., the thrust load is located at zero angle and swing, respectively), the optimized result is that the large wing plate structure between the middle thrust load area and the connection position of conical structure is constructed to enhance the structural bending capacity. Compared with the traditional frame structure, the effectiveness of the proposed method in the lightweight design is proved.