A　continuous-flow　anaerobic/aerobic/anoxic　reactor　with　complete　suspended　activated　sludge　using　sludge　alkaline　fermentation　products　as　carbon　source　was　utilized　to　strengthen　nitrogen　removal　performance　for　low　C/N　ratio　(＜4)　wastewater.　Long-time　performance　indicated　that　the　nitrite　accumulation　rate　reached　60.40%,　which　strengthened　the　contribution　of　anammox.　The　average　total　inorganic　nitrogen　removal　efficiency　improved　19.40%.　The　abundance　of　ammonia　oxidizing　bacteria　has　not　changed,　but　the　abundance　of　nitrite　oxidizing　bacteria　reduced　from　5.79%　to　0.69%.　Quantitative　PCR　results　demonstrated　that　the　abundance　of　anammox　bacteria　has　raised　by　80.5　times.　These　results　indicated　that　side-stream　sludge　alkaline　fermentation　promoted　the　mainstream　partial　nitrification,　consequently　accelerating　the　in-situ　enrichment　of　anammox　bacteria.　No　external　carbon　source　dosing　and　short　oxic　hydraulic　retention　time　(5.3　h)　save　energy　and　reduce　consumption　significantly　in　this　system.