In order to avoid large-scale casualties caused by gas leak in short time, this article which based on the main diffusion model of poison gas analyze the diffusion principle of poison gas, and choose suitable leakage and diffusion model. After that, according to poison load norm calculation formula and the condition of real-time parameter, the article carried out the partition calculation of dangerous area after diffusion of poison gas which based on GIS. Finally, to make the dangerous area analysis result of poison gas intuitive and practical, this thesis simulate the display process, including calculation parameters input, module selecting calculation, the layer formation and results show.