Abstract: The impact of outdoor air pollution on the indoor air quality by the way of numerical simulation was studied. Firstly the simulation models and methods were proved by using the wind tunnel test of point source air pollution diffusion around an isolated building, and then the outdoor and indoor concentration fields were simulated with different source positions and inlet conditions, whosevelocity fields were verified well by the wind tunnel test for the natural ventilation. The results showed that dimensionless ventilation rates were a little smaller than the test and the building inlet flows were from the upper zone of the backflow in front of the building. The indoor concentration was lower with thesource on the ground than the source as high as one half of the building height. The windbreak panel had little effect on decreasing the indoor concentration, while adopting the low concentration inlet was more effective for the different pollution distribution on the building walls.