The large arm of ER20-C10 six degree-of-freedom industrial robot was taken as the research object. In view of the demand situation of light weight, high strength and good dynamic characteristics, on the basis of ensuring the existing function of the arm, the optimal structure of the arm was studied by orthogonal experimental design. Through many orthogonal tests, the value range of each influencing factor was constantly reduced, and finally the optimal arm structure was confirmed. ANSA software was used to clean up the geometry and divide the grid of the optimized arm, solidThinking Inspire software was used to analyze the static performance of the optimized arm, and ANSYS Workbench was used to make the modal analysis of the optimized arm. Finally, the analysis results were compared with those of the original model. It is found that not only the optimized arm strength, stiffness and minimum safety factor are improved, but also the mass is reduced. The lightweight research for the big arm greatly reduces the mass of the big arm, at the same time, the maximum stress is smaller than that of the optimized arm.It provides reference for the research and development of the mechanical structure of the industrial robot.