A　modification　of　the　two　sludge　A2/O-BAF　system　was　used　to　treat　low　C/N　real　domestic　sewage.　In　order　to　improve　the　utilization　of　the　carbon　source,　the　effects　of　two　step　feeds　(pre-anoxic　zone　and　anoxic　zone)　on　denitrifying　phosphorus　and　nitrate　removal　were　studied.　According　to　the　formula　of　material　balance　for　COD,　the　utilization　of　carbon　source　was　analyzed　and　evaluated　under　different　ratios　of　step　feed,　simultaneously.　The　results　showed　that　when　the　ratio　of　step　feed　was　7:3　and　the　influent　concentrations　of　COD,　NH4+-N,　TN,　and　TP　were　174.99,　58.19,　59.10,　and　5.15　mg•L-1,　respectively,　their　effluent　concentrations　were　29.48,　4.07,　14.10,　and　0.40　mg•L-1,　and　the　removal　rates　were　82.12%,　92.76%,　75.45%,　and　91.20%,　respectively.　It　was　found　that　when　the　ratio　of　the　denitrifying　phosphorus　accumulation　organisms　to　the　phosphorus　accumulation　organisms(DPAOs/PAOs)　was　98.81%,　the　efficiencies　of　denitrifying　phosphorus　and　nitrate　removal　were　optimum.　By　optimizing　step　feed,　the　carbon　source　was　utilized　effectively,　and　the　efficiencies　of　nitrogen　and　phosphorus　removal　were　improved　simultaneously.　The　theoretical　basis　has　thus　been　provided　for　the　modification　of　the　two　sludge　A2/O-BAF　system　to　treat　low　C/N　waste　water.　©　2018,　Science　Press.　All　right　reserved.