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.