Aerosol　samples　of　PM2.5　were　collected　in　Beijing　during　January,　April,　July　and　October　2014,　which　stood　for　winter,　spring,　summer　and　autumn　respectively,　to　investigate　the　physical　and　chemical　characteristics,　seasonal　variation　and　formation　factors　of　PM2.5.　At　the　same　time,　source　apportionment　was　carried　out　by　using　the　positive　matrix　factorization　analysis　method.　The　results　showed　that　the　annual　average　concentration　of　PM2.5　reached　87.74　μg/m3　in　Beijing　in　2014,　2.5　times　as　high　as　the　national　air　quality　standard.　The　PM2.5　concentrations　during　light　and　heavy　pollution　periods　increased　more　than　1.5　and　3.9　times　compared　with　normal　days,　respectively,　and　its　seasonal　variation　was　significant,　with　the　order　of　winter＞summer＞autumn＞spring.　The　concentrations　of　crustal　elements,　such　as　Mg,　Al,　Fe,　Ca,　Ti,　during　light　and　heavy　pollution　days　were　1.1~1.2　and　1.2~1.5　times　as　high　as　those　in　normal　days,　respectively,　and　the　concentrations　of　pollution　elements,　such　as　S,　Pb,　Zn　and　Cu,　showed　significant　changes　during　light　and　heavy　pollution　days,　with　1.3~2.7　and　1.9~5.9　times　as　high　as　those　in　normal　days,　respectively.　S　was　the　most　important　anthropogenic　pollution　element　in　PM2.5,　and　its　SO42-　annual　average　concentration　was　13.43　μg/m3.　The　concentrations　of　SO42-　in　light　and　heavy　pollution　days　were　2.7　and　5.9　times　higher　than　that　in　normal　days,　respectively.　The　formation　of　sulfate　was　mainly　influenced　by　the　concentration　of　O3,　air　temperature　and　relative　humidity　etc..　Higher　O3　concentration,　temperature　and　relative　humility　were　in　favor　of　the　formation　of　sulfate.　PM2.5　was　mainly　from　mobile　emission,　coal　combustion,　re-suspended　dust　and　industrial　emission,　which　contributed　to　PM2.5　about　37.6%,　30.7%,　16.6%　and　15.1%,　respectively.　©　2016,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.