The invention relates to a component topology optimization design method for a complex assembly, and aims to solve the problem of difficulty in the determination of boundary conditions in a topology optimization design process for components of the complex assembly. The method comprises the following steps of: establishing and analyzing an assembly finite element model, and extracting and exporting the boundary conditions of a sub-structure which is a component to be optimized in the assembly model; loading the extracted boundary conditions onto the component to be optimized, and performing topology optimization on the component by utilizing a density method; when a change in the size of an optimization object exceeds a preset value, re-extracting the boundary conditions of the component, and continuing performing the topology optimization on the component; and when the optimization object is consistent with an iteration stopping condition, stopping optimization iteration to obtain a final optimal structure. An accurate optimal structure of the component can be obtained by accurately determining the boundary conditions of the component, and the problems of a plurality of design variables, inaccurate contact definition of combination parts between components and large calculated amount caused by the direct adoption of the assembly for the topology optimization design are solved.