Abstract: Based on a Lagrangian particle dispersion model FLEXPART 3-D in conjunction with a weather research and forecasting (WRF) model,this study investigates the mechanism and trajectory of pollutants under the anti-cyclonic weather system that took place during October 18-22,2003 in Beijing.The reasons for this heavy air pollution episode were also analyzed by using the environmental and meteorological observational data.The findings indicate that pollutants hovered in Beijing by local wind under the control of anticyclone,the accumulated pollutants was transported to the northeast during the back of anticyclone,and the pollutants finally converged to the low-pressure area and elevated to free atmosphere.