In　this　paper,　a　coupled　MM5-CAMx　modeling　system　and　meteorological　observation　data　were　used　to　analyze　a　PM10　air　pollution　event　in　Beijing.　The　PSAT　technique　was　applied　to　source　apportion　Beijing　PM10　contributions　to　source　emission　from　different　regions　in　North　China.　The　result　of　modeling　system　indicated　that　PM10　trans-boundary　transportation　especially　the　emission　from　Hebei　and　Tianjin　was　the　trigger　of　this　PM10　pollution　event　in　Beijing.　Meteorological　observation　diagnose　confirmed　that　this　PM10　pollution　event　in　Beijing　was　influenced　by　regional　meteorological　factors,　and　concluded　that　the　high　PM10　concentration　process　in　this　event　could　be　divided　into　two　phases.　In　phase　1,　trans-boundary　transportation　by　southwest　wind　which　increased　the　PM10　concentration　in　Beijing　was　the　cause　of　this　PM10　pollution　event.　In　phase　2,　even　pressure　and　weak　low　pressure　field　which　caused　the　PM10　convergence　from　Beijing　local　emission　maintained　this　process　of　high　PM10　concentration.　In　addition,　PM10　trans-boundary　transportation　from　Tianjin　by　southeast　wind　also　promoted　the　high　PM10　concentration　in　phase　2.