By　treating　real　domestic　sewage　which　is　low　ρ(C)/ρ(N)　in　sequencing　batch　reactor　(SBR),　the　cause　of　filamentous　sludge　bulking　and　the　effect　of　controlling　filamentous　sludge　bulking　under　low　dissolved　oxygen　with　different　operation　patterns　were　investigated.　Results　showed　that　when　the　dissolved　oxygen　(DO)　value　was　0.5　mg/L　in　anoxic/oxic　process　(A/O),　filamentous　sludge　bulking　happened　and　H.　hydrossis　was　the　dominant　filamentous　bacteria　in　filamentous　bulking　sludge.　During　A/　O　pattern　in　SBR,　sludge　volume　index　(SVI)　has　maintained　beyond　300　mL/g　when　organic　loading　(F/M)　was　0.83　kg/(kg·d)　and　the　ρ(DO)　was　about　2　mg/L,　the　sludge　bulking　could　not　be　effectively　controlled.　The　result　of　fluorescence　in　situ　hybridization　(FISH)　manifested　that　the　main　filamentous　bacteria　was　also　H.hydrossis　filamentous　bacteria.　In　this　period,　average　removal　rates　of　total　nitrogen　(TN)　and　COD　were　31.17%　and　65.04%,　respectively.　Then　during　aerobic　pattern,　when　F/M　was　0.37　kg/(kg·d),　the　ρ(DO)　was　also　about　2　mg/L,　the　sludge　bulking　has　been　controlled　effectively　and　SVI　has　remained　below　150　mL/g.　In　this　stage,　the　removal　rate　of　NH4+-N　reached　100%　nearly,　the　ρ(NH4+-N)in　effluent　was　almost　0.　Average　removal　rates　of　TN　and　COD　improved　to　70.7%　and　73.9%,　respectively.　The　average　ρ(TN)　and　ρ(COD)　in　effluent　were　18.4　and　46.9　mg/L,　respectively,　close　to　the　national　level　A　emission　standard.