To　commemorate　the　70th　anniversary　of　the　victory　of　the　Chinese　people＇s　Anti-Japanese　War　and　the　World　AntiFascist　War,　an　international　parade　was　held　in　Beijing　in　September　2015.　In　order　to　ensure　satisfactory　air　quality　during　this　event,　a　phased　emission　control　measures　were　taken　in　Beijing　and　its　surrounding　provinces.　The　24-h　PM2.5　samples　were　collected　in　Beijing　from　August　1　to　September　15,　2015　covering　the　period　before,　during　and　after　this　large-scale　event.　The　observed　PM2.5　data,　meteorological　data,　emission　reduction　measures,　and　air　mass　trajectory　simulation　results　were　systematically　analyzed　to　understand　the　pollution　characteristics　and　chemical　compositions　of　PM2.5　in　Beijing.　The　results　indicated　that　PM2.5　concentration　during　the　two　emission　control　phases　was　reduced　by　61.7%　comparing　to　the　non-control　period,　but　the　regional　transport　of　pollutants　and　meteorological　conditions　had　a　more　prominent　impact　on　PM2.5　than　emission　reduction　during　phase　2.　The　secondary　water-soluble　ions　including　SO42-,　NO3-,　and　NH4+　were　found　as　the　main　ions　present　in　PM2.5.　During　the　entire　emission　control　period,　organic　carbon　(OC)　and　elemental　carbon　(EC)　mass　concentrations　were　decreased　by　53.1%　and　57.9%.　A　PM2.5　mass　balance　was　analyzed,　and　it　was　found　that　the　organic　matter　accounted　for　29.3,　37.6　and　28.5%　of　the　PM2.5　mass　before,　during　and　after　the　emission　control,　while　the　contribution　of　mobile　sources　to　PM2.5　was　relatively　outstanding　after　a　series　of　emission　control　measures.