Abstract: The Mesoscale Model (MM5) and NOAA Back Trajectory Model (HYSPLIT-4) were used to simulate and analyze an air pollution event occurred from October 7th to 13th, 2002. The results show the formulation mechanism and development process of the event, and indicate that the pollution event was of regional characteristics and resulted from combined influences of local pollution sources and emissions out of Beijing in special weather conditions. According to back trajectory analysis based on HYSPLIT, the pollution in the surrounding areas of Beijing has significant impacts on Beijing, with the import of pollutants from the cities to the south of Beijing being the main sources.