This paper studies a BIM-based virtual construction simulation system for large steel structures and its application in construction project. Environmental complexity, high requirement for operation and multiple potential safety hazards are the basic features of the construction for large steel structures, which are difficult to be considered thoroughly. Aiming at solving those difficulties, the virtual construction simulation system integrates Unity3d and BIM, which can fully utilize and visualize the information of construction design. It is able to help build a virtual construction site scene which vividly displays various circumstances, and can be applied for construction management, technical disclosure, as well as thorough check for potential quality and safety hazards. The system has been successfully applied in the large-span steel truss construction process of Hangzhou Shangcheng District Sport Center project, which evidently enhanced construction efficiency and fully guaranteed the quality and safety in construction process.