The mechanical excavator is the essential equipment for open-pit mining. The rotary platform is main bearing part of the excavator, stiffness and strength are directly related to the safety of the excavator. The rotary platform of a mechanical excavator is applied as the research object. The Shell181 and Solid 95 unit are used to establish finite element model based on analyzing the structural characteristics. MPC technology is applied to solve the coupling problem of the shell and the solid element. Taking the total weight of platform as the objective function of optimization, stiffness and strength condition as constraint conditions, the section thickness of the platform main components as the design variables, the structural optimization of the platform has been completed, and the weight decrease by 6.55%. A new way has been explored to make the design of platform structure more economic and reasonable, and improve the design efficiency.