A three-dimensional numerical simulation is used to predict the exhaust gas diffusion from a tunnel portal under different wind velocity and direction. The results show that the higher the wind velocity is, the faster the concentration of the pollutants will attenuate after the pollutants leave the tunnel portal;while the transversal wind will result in the higher concentration of pollutants in a large range around the tunnel portal. So effective methods should be taken to control the diffusion of the pollutants from the tunnel portal.