On the basis of analyzing the structure of planetary gear reducer, this paper points out that it’s necessary to make use of optimization design method to improve the design level of the planetary gear reducer. The optimization model of planet carrier is established and the process of optimization design is studied. Taking the XJB80 single arm planet carrier as an example, whose optimization design is performed by using ANSYS software. The spacing of connecting panel, thickness of sidewall, the inner radius of the connecting panel and some other parameters are selected as design variables, the yield strength and structural deformation limitation of planet carrier material are selected as state variables and the total volume is selected as objective function. The optimal structure size of the planet carrier is obtained, result shows that the corresponding volume is reduced by 21%, thus the manufacturing cost can be reduced. The study result has already been used by planetary gear reducer enterprise.