Virtual assembly (VA) is an important application for virtual reality (VR) technology in manufacturing industries that can supply an effective solution to actual assembly problem of products. According to the assembly characteristic of large-scale products such as airplane, rocket and satellite, etc, a new interactive VA environment system is constructed, and its architecture and related key techniques are studied. Considering human free activity in the system, a free-walking perception machine is developed. And a stereo projection approach based on a video generator is proposed to enhance real-time interaction and reduce hardware cost. Finally, those key techniques are successfully applied in the process of satellite assembly planning and training.