In　recent　years,　people　are　facing　the　adverse　impacts　of　the　fine　particles　(referred　as　PM2.5)　in　Beijing,　which　will　cause　great　harm　to　human　health.　In　this　study,　based　on　a　large　number　of　indoor　and　outdoor　PM2.5　mass　concentrations　field　monitored　data　under　the　different　air-tightness　level　of　external　windows　and　mathematical　statistics　method,　a　PM2.5　pollution　situation　with　different　air-tightness　levels　of　windows　is　studied.　Moreover,　as　considering　of　individuals　spend　the　majority　spare　time　indoors,　the　particles　mass　concentrations　is　also　investigated　with　different　window　air-tightness　levels　under　infiltration.　This　study　can　provide　a　reference　for　the　optimization　design　and　equipment　selection　of　the　ventilation　purification　system,　as　well　as　the　operation　and　maintenance　management　of　the　system.　(C)　2017　The　Authors.　Published　by　Elsevier　Ltd.