A　campaign　of　sampling　atmospheric　submicron　particulate　matter　(PM1)　samples　at　Yizhuang　zone　in　Beijing　during　July,　October　2016　and　January,　April　2017,　was　carried　out　to　investigate　the　characteristics　of　PM1　and　its　water-soluble　species　during　four　seasons　and　different　haze　periods.　The　important　factors　affecting　the　formations　of　the　secondary　ions　and　haze　pollution　were　discussed.　The　results　showed　that　the　average　concentration　of　PM1　reached　73.95μg/m3　for　the　entire　study　at　Yizhuang　zone,　being　1.13times　as　high　as　the　corresponding　average　concentration　level　of　PM1　in　Beijing.　Average　concentrations　of　PM1　in　summer,　autumn,　winter　and　spring　were　69.22,　63.38,　99.50　and　57.26μg/m3,　respectively,　showing　the　order　of　winter＞summer＞autumn＞spring;　the　concentrations　of　PM1　during　haze　days　were　1.78~3.17times　as　high　as　those　in　clean　days.　The　total　water-soluble　ion　concentration　in　PM1　was　37.30μg/m3,　accounting　for　50.44%　of　PM1;　secondary　ions　SO42-,　NO3-　and　NH4+　(SNA)　were　the　most　important　water　soluble　ions,　which　accounted　for　86.98%　of　the　total　water　soluble　ions.　The　seasonal　variation　of　the　total　water　soluble　ion　concentrations　in　PM1　was　in　accordance　with　SNA,　following　the　order　of　winter　＞　summer　＞　autumn　＞　spring.　The　average　sulfur　oxidation　rate　SOR　was　higher　than　the　mean　nitrogen　oxidation　rate　NOR　for　the　entire　study,　of　which　SOR　presented　the　order　of　summer　＞　autumn　＞　winter　＞　spring,　and　NOR　showed　the　order　of　summer　＞　autumn~spring　＞　winter,　and　SORs　and　NORs　in　haze　days　were　all　significantly　higher　than　those　in　clean　days,　especially　in　summer.　The　conversion　of　SO2　to　SO42-　was　apparently　influenced　by　relative　humidity　RH,　temperature　T,　NO2　and　NH3,　and　aqueous　chemical　reactions　of　SO2　on　the　surface　of　PM1　might　be　the　important　pathway　of　SO42-　formations.　The　conversion　of　NO2　to　NO3-　was　greatly　influenced　by　RH,　T,　O3,　and　NH3.　The　haze　pollution　formation　was　mainly　influenced　by　saddle　type　pressure　field,　even　pressure　field,　and　inversion　layer,　as　well　as　weak　air　mass　transport　from　the　south,　southeast　and　southwest　directions.　©　2018,　Science　Press.　All　right　reserved.