Due to complex process and high difficult operation for large-scale products assembly, the application of virtual assembly (VA) has an important significance and also an actual value. In this paper the key techniques of VA for large-scale complex products are proposed, and their assembly characteristics (such as their organization form, human participation, process planning and optimization and so on) are analyzed. First, a new virtual environment (VE) system considering human activities is set up. Second, an integration approach is realized to transform data from CAD to VA system, and a hybrid method on the basis of virtual reality (VR) and intelligent algorithm is applied to optimize and evaluate assembly process planning. Moreover, assembly documents generation and Web training for assembly workers are implemented. Finally, an application case of the VA system for a complex product is given, and further work is pointed out.