Abstract: Efficient emergency rehearse system provides both commanders and evacuation crowds with better preparation for dangerous gas diffusion accidents, which definitely causes serious social panic and huge property loss. This paper focuses on four key issues raised by such accidents in the chemical industry parks. These issues include modeling, simulation, rendering and decision making. Virtual Reality and GIS are exploited to enhance intuitive and comprehensive visual information. Google SketchUp is worked as large model resource library and used to efficiently assemble the targeted virtual environments. Simulation of dangerous gas diffusion is built based on Pasquill-Gifford model and gas properties. Puff model combined with plume model is used to generate the gas diffusion animation. And evacuation route of crowds is display through scene-GIS map. Large scale virtual environment is rendered using OSG engine. All the simulation processes are recorded in a video with time tag embedded, which enable the commanders review history scene and hence make reasonable decision instantly. The gas emergency rehearse system enable all the people involved in this system having useful information and immersive experience. The system is now integrated into a common public safety platform and running well.