Abstract: A system model is constructed in laboratory based on similarity theory and a model experiment is carried out using tracer technique. Having obtained experimental data, qualitative and quantitative analysis are performed to the diffusion rule of the harmful gas. The results of qualitative analysis showed that there exist two similarity criterions: and, their corresponding criterion is; the diffusion equations are deduced by quantitative analysis, and the mathematical representation is obtained; The research has a certain reference value to trend prediction, establishment of emergency response rescue plan and implementation of effective field control.