A　pilot-scale　step-feed　A/O　system　was　used　to　treat　low　C/N　municipal　wastewater.　To　obtain　high　quality　simultaneous　nitrogen　and　phosphorus　removal,　a　modified　step-feed　A/O　system　was　developed　and　the　nitrogen　and　phosphorus　removal　efficiency　was　evaluated　between　the　original　and　modified　system.　The　result　showed　that　TN　removal　efficiency　was　66.52%　and　TP　removal　efficiency　was　only　29.24%　before　the　modified　configuration　was　applied.　When　the　modified　system　was　adopted,　the　satisfied　simultaneous　nitrogen　and　phosphorus　removal　was　achieved.　TP　removal　efficiency　increased　to　89.81%　and　TN　removal　efficiency　was　73.61%　when　the　optimal　feeding　ratio　(0.45:0.35:0.20)　was　applied.　Compared　to　the　original　configuration,　TN　removal　efficiency　increased　about　7.09%　due　to　the　enhanced　denitrifying　phosphorus　removal,　which　saved　the　carbon　source　for　denitrification.　To　evaluate　the　selection　and　dominance　of　polyphosphate-accumulating　organisms　(PAOs)　and　denitrifying　polyphosphate-accumulating　organisms　(DNPAOs),　batch　test　was　explored　to　examine　the　anaerobic　phosphorus　release,　aerobic　and　anoxic　phosphorus　uptake.　The　result　showed　that　both　PAOs　and　DNPAOs　were　accumulated　gradually　when　the　modified　system　was　applied.　The　maximal　aerobic　P　uptake　rate　[P/(MLSS　x　t)]　was　increased　from　2.34　mg/(g　x　h)　to　10.67　mg/(g　x　h)　and　the　anoxic　P　uptake　rate　was　increased　from　0.33　mg/(g　x　h)　to　2.81　mg/(g　x　h)　when　the　modified　system　was　operated.