Outdoor　Fine　Particular　Matter　(PM2.5)　can　enter　the　room　through　both　ventilation　and　infiltration,　and　this　will　make　indoor　air　polluted.　Penetration　factor　(P),　deposition　loss　rate　(k)　and　air　exchange　rate　(a)　are　important　parameters　that　can　be　used　to　evaluate　the　degree　of　indoor　air　pollution　caused　by　PM2.5.　Currently,　these　parameters　were　mostly　derived　from　either　laboratory　experiment　or　theoretical　deduction.　According　to　the　law　of　conservation　of　mass,　statistical　method　and　control　variable　method,　a　new　model　estimating　the　above　three　parameters　was　developed　in　this　study,　based　on　a　big　number　of　field　monitored　data,　and　the　model　can　be　used　for　a　variety　of　applications.　The　model　has　been　used　to　calculate　the　penetration　factor　and　deposition　loss　rate　at　two　case　study　office　buildings　and　the　prediction　gave　0.97　(P)　and　0.12　(k),　and,　0.97　(P)　and　0.24　(k),　respectively.　This　model　can　be　considered　as　a　new　method　for　studying　the　penetration　characteristics　of　external　windows,　the　correlation　between　outdoor　and　indoor　PM2.5　pollutions,　and　the　quantification　of　indoor　concentration　of　PM2.5.　©　2016,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.