The invention provides a lightweight design method of a leg structure of a biped humanoid robot. The lightweight design method comprises the following steps: (1) performing evolutionary structural topological optimization based design on the leg structure of the robot according to the weight bearing ability of the leg structure of the robot, and setting two working conditions, namely an upright state in which the included angle of the body and the legs is 180 degrees and an bent state in which the included angle is 150 degrees, for finite element analysis, (2) performing meshing, freezing fixed and part installed components and bolt and sliding groove components with the strain energy of the sum of two load working conditions as a target function and the volume as a constraint condition, adopting withdrawing pattern constraint in the vertical direction of a cover plate and performing topological structural optimization, and (3) designing the legs of the humanoid robot according to the optimization result. The lightweight design method of the leg structure of the biped humanoid robot is capable of realizing the optimal distribution of materials and reducing the outline and the quality of the leg structure of the biped humanoid robot by virtue of the evolutionary structural topological optimization.