A　two-sludge　system　combined　anaerobic/anoxic/oxic　with　moving　bed　biofilm　reactor　(A2/O-MBBR)　was　used　to　treat　real　domestic　wastewater.　By　adding　sodium　acetate　to　adjust　influent　carbon/nitrogen　ratio　(C/N),　the　transformation　and　utilization　and　metabolic　pathways　of　organic　matter　in　the　system　were　investigated.　When　the　influent　C/N　ratio　varied　from　3.11　to　6.09,　the　system　all　achieved　efficient　COD　removal　with　the　maximum　removal　of　83.85%.　However,　from　the　perspective　of　VFA　and　PHAs　transformation,　the　influent　C/N　ratio　should　not　exceed　5.42.　Additionally,　the　material　balance　analysis　of　carbon　also　revealed　that　when　the　influent　C/N　ratio　was　below　5.42,　COD　removal　percentage　in　the　A2/O　reactor　was　up　to　70.10%　and　that　in　the　MBBR　reactor　only　accounted　for　4.32%~5.89%,　where　the　efficient　utilization　of　carbon　source　was　the　key　to　improve　microbial　community　optimization　and　enhance　denitrification　and　phosphorus　removals　under　the　condition　of　low　C/N　ratio.　The　results　of　dissolved　carbon　source　component　distribution　and　three-dimensional　fluorescence　properties　manifested　that　proteins　and　polysaccharides　transformed　although　COD　changed　gently　along　the　reactor,　in　which　proteins-like　substance　was　the　dominant.　©　2017,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.