A lightweight design method for an arm structure of an assembly robot comprises combining simulation of many-body dynamics and simulation of finite elements to obtain the maximal load condition of stress during a period of motion of an arm, and designing the load condition through topological optimization on a structure to obtain the maximal lightweight design of an arm structure. The lightweight design method for an arm structure of an assembly robot is advantageous in that the load obtained by simulation of many-body dynamics is used as the load of a structural optimal design, so as to raise the accuracy of the structural optimal design; and the speed of motion of an assembly robot can be raised by lightening weights of a large arm a small arm of the assembly robot, so as to raise the assembling efficiency. The arm structure design obtained by the method is high in accuracy and good in optimization.