Abstract: Diffusion of polluting gas outside urban tunnel portal has always been closely watched. In order to study the influence of polluting gas, a tunnel in Chongqing is set as an example, which is based on the diffusion theorem. In this paper, it’s used COMSOL Multiphysic software to establish a three-dimensional model which simulates the diffusion of polluting gas outside tunnel portal at low wind speed. The simulation result shows that the influence of gas diffusion along the tunnel axis direction is greater than that to the both sides of road and upper side. Along the tunnel axis direction, it’s about 250m away from the tunnel portal that the concentration of CO decays to the specification requirements. However, to the both sides of road and upper side, it’s about 30m that the concentration of CO has decayed to the specification requirements.