At　normal　temperature,　short-cut　nitrification　and　denitrification　was　achieved　in　a　lab-scale　A2O　process　treating　low　C/N　ratio　domestic　wastewater　by　controlling　DO　concentration　of　0.3-0.5　mg/L　and　increasing　the　internal　reflux　ratio　to　decrease　the　actual　aerobic　HRT.　However,　the　phosphorus　removal　in　A2O　process　was　deteriorated　with　the　increasing　of　the　nitrite　concentration　in　the　effluent.　The　factors　causing　phosphorus　removal　deterioration,　such　as　the　influent　COD　concentrations,　temperature,　pH　and　free　nitrous　acid　(FNA)　were　systematically　analyzed.　Experimental　results　showed　that　the　nitrite　accumulation　resulting　from　short-cut　nitrification　affected　anaerobic　P　release　and　aerobic　P　uptake.　Especially,　the　higher　FNA　concentration　(HNO2-N　0.002-0.003　mg/L)　in　the　aerobic　zone　significantly　inhibited　the　aerobic　P　uptake,　which　was　the　major　reason　causing　P　removal　deterioration.　Through　adding　the　carbon　sources　in　influent　to　enhance　anaerobic　P　release　and　denitrification,　the　nitrite　and　FNA　concentrations　in　the　aerobic　zone　were　decreased,　and　the　P　removal　was　recovered.　More　than　96%　of　PO43--P　could　be　removed　in　A2O　process.