A2O　and　A2O+BCO　process　were　employed　to　treat　domestic　wastewater　with　low　carbon　to　nitrogen　ratio　(C/N=3.96)　to　compare　their　nutrient　removal　performance,　respectively.　The　influent　loading　and　operation　condition　were　kept　consistent　for　134　days,　A2O+BCO　system　achieved　a　higher　nitrogen　and　phosphorus　removal　efficiency　than　A2O　system　because　of　the　application　of　denitrifying　phosphorus　removal　technology　and　two-sludge　theory,　which　could　use　carbon　resource　more　efficiently　and　solve　the　SRT　contradiction　between　nitrifiers　and　PAOs.　PAOs　accounts　for　22%　in　the　stable　A2O+BCO　system　higher　than　7%　in　the　A2O　system,　which　explain　the　mechanism　for　the　superior　nutrient　removal　performance.　©　2019,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.