In　the　pre-denitrification　of　anoxic-aerobic　reaction　process,　nitrite　accumulation　and　sludge　performance　at　different　phenol　concentrations　were　investigated.　The　results　showed　that　there　were　two　obvious　nitrite　accumulations　finally　stabilized　at　70%±5%　when　phenol　varied　from　0　to　90　mg·L-1.　At　low　phenol　concentration　(0　to　30　mg·L-1),　nitrite　accumulation　and　recovery　were　caused　by　microbial　structure　varieties　and　secreting　extracellular　polymer.　At　high　phenol　concentration　(60-90　mg·L-1),　nitrite　accumulation　was　caused　by　microbial　population　changes　of　ammonia-oxidizing　bacteria　and　nitrite-oxidizing　bacteria　was　caused　by　phenol　inhibition.　Ammonia　oxidation　rates　and　nitrogen　oxide　production　rates　varied　from　10.85　mg　N·(g　MLSS)-1·h-1　and　10.12　mg　N·(g　MLSS)-1·h-1　to　2.79　mg　N·(g　MLSS)-1·h-1　and　2.32　mg　N·(g　MLSS)-1·h-1,　respectively.　Nitrite　accumulation　rate　had　a　negative　correlation　with　nitrogen　oxide　production　rate,　whereas　a　positive　correlation　to　phenol　concentration.　Fluorescence　in　situ　hybridization　showed　that　nitrifying　bacteria　communities　changed　due　to　phenol　inhibition,　with　AOB　relative　quantity　increasing　from　2.80%　to　9.30%.　As　carbon　source,　phenol　caused　sludge　concentration　increase　from　2500　mg·L-1　up　to　5870　mg·L-1.　When　total　extracellular　polymeric　substances　declined　from　67.20　mg·(g　VSS)-1　to　32.10　mg·(g　VSS)-1,　sludge　volume　index　(SVI)　accordingly　changed　from　165　ml·g-1　to　50　ml·g-1.　Nitrite　accumulation,　filamentous　bacteria　and　extracellular　polymer　contributed　to　SVI　changes　in　the　activated　sludge　system,　and　nitrite　accumulation　was　the　main　reason　followed　by　filamentous　bacteria.　©　All　Rights　Reserved.