Human crowd simulation is a new technology in the virtual reality field. Since it could simulate evacuation, it has strong demands in risk assessment for public buildings. In this paper we discuss the development of the main related research topics, including semantic description for virtual environments and crowd models which generate continuum human flow. Additionally, we introduce a system named Guarder that is designed for human crowd simulation and is suit for simulating evacuation in public buildings. We also demonstrate some simulation results of Guarder to show that it could efficiently simulate evacuation in a large-scale and complex environment.