This paper introduces a virtual simulation method of gas diffusion in the Chemical Industry Park (CIP), the CIP scene in 3D was established on the basis of the real terrain. To ammonia leakage and diffusion for example, the process of leakage accident was realized in virtual reality by means of embedded the toxic gas diffusion mathematical model to 3D scene. The whole process of the accident can be viewed from a different perspective. The entity model in 3D scene was created by Creator, the construction of the accident scene and render was accomplished by the Vega API. The special effects in the scene were achieved with the help of particle system technology. The research results show that the virtual simulation method can effectively simulate accidents in the 3D scene and it is great significance during the emergency plan drills and accident process analysis.