The arm structure of the industrial robot is the main bearing structure of the mechanical arm, and the mass is relatively large. In this paper, the arm structure is selected for lightweight research. Firstly, the structure of the original steel arm is analyzed by using finite element simulation software to obtain its stiffness and displacement. Then, the carbon fiber-reinforced thermoplastic polymer composite boom structure is designed and optimized according to the principle of equal stiffness. In the finite element model of the arm, the steel material is replaced by thermoplastic carbon fiber composite material, and it is found that the weight of the arm structure is greatly reduced and the original stiffness is not reduced. On this basis, the concept of SMEAR super layer is used to carry out multi-level optimization design, which is optimized step by step from free size, layer thickness, and layer order, and the quality of the composite mechanical arm is further reduced, which verifies the effectiveness of the lightweight design method.