Experiment　operated　through　dosing　nitrite　to　aerobic　and　anoxic　conditions　respectively　in　A2O　system.　Effect　of　nitrite　on　nitrogen　and　phosphorus　removal　was　analyzed.　Results　showed　that　nitrification,　denitrification　and　phosphorus　removal　performance　of　the　system　were　more　sensitive　to　the　presence　of　nitrite.Nitrite　existed　in　aerobic　period　had　more　influence　on　the　performance　of　nitrification,　when　nitrite　concentration　in　aerobic　period　reached　to　25　mg/L,　nitrification　rate　of　the　system　was　only　5.26　mg/(L·h).　Nitrite　existed　in　anoxic　period　had　more　inhibition　on　denitrification　performance,　when　nitrite　existed　in　anoxic　period　for　long　time,　denitrification　rate　of　the　system　down　to　11.83mg/(L·h)　that　fall　by　60%　compared　with　normal　condition.　Nitrite　existed　in　aerobic　period　could　inhibit　the　phosphorus　absorption　ability　of　phosphorus-accumulating　bacteria　seriously　with　phosphorus　removal　rate　of　the　system　was　only　22%;　When　nitrite　existed　in　anoxic　period,　it　could　cause　sludge　bulking　problem　in　the　system,　lead　to　the　loss　of　phosphorus-accumulating　bacteria,　the　number　of　phosphorus-accumulating　bacteria　decreased　to　about　2.02%,　which　in　turn　trigger　phosphorus　removal　performance　deteriorated　seriously.