Based　on　atmospheric　monitoring　data　and　the　WRF-CAMx　model,　this　study　analyzed　the　characteristics　of　air　pollution　and　performed　a　quantitative　assessment　of　PM2.5　cross-border　transport　in　the　Beijing-Tianjin-Hebei　(BTH)　region　in　January　2016.　The　results　showed　that　the　average　concentrations　of　PM2.5,　PM10,　SO2,　NO2,　and　CO　were　89.5　μg·m-3,　135.61μg·m-3,　57.55μg·m-3,　60.79μg·m-3,　and　2.12　mg·m-3,　respectively,　indicating　severe　PM2.5　pollution.　During　the　study　period,　surface-level　PM2.5　in　each　city　of　BTH　region　was　dominated　by　local　emissions,　which　accounted　for　45.4%　to　69.9%.　The　regional　transport　contribution　was　supplemented　by　transport　from　within　and　outside　of　the　BTH　region,　accounting　for　4.8%　to　49.7%　and　4.9%　to　29.6%,　respectively.　In　addition,　high　wind　speeds　promoted　the　diffusion　of　local　PM2.5　pollution　and　cities　with　high　upwind　pollution　enhance　regional-scale　transport　to　downwind　cities.　The　total　inflow,　outflow,　and　net　flux　of　PM2.5　in　Beijing　(Shijiazhuang)　in　January　2016　were　1582.96　t·d-1　(2036.89　t·d-1),　-1171.09　t·d-1　(-1879.12　t·d-1),　and　411.87　t·d-1　(157.77　t·d-1),　respectively,　indicating　that　PM2.5　inputs　from　surrounding　cities　per　unit　time　were　higher　than　external　inputs　to　the　surrounding　cities.　Furthermore,　net　PM2.5　flux　showed　notable　vertical　evolution;　the　total　net　flux　of　PM2.5　in　Beijing　and　Shijiazhuang　below　1782　m　ranged　from　17.86　to　64.18　t·d-1　and　-2.95　to　134.81　t·d-1,　respectively,　and　both　peaked　817　m　above　the　ground　at　64.18　and　134.81　t·d-1.　Moreover,　a　significant　increase　the　net　PM2.5　inflow　flux　in　Zhangjiakou　and　Shanxi　explained　the　observed　net　flux　peaks　in　these　two　cities.