The　long-term　effects　on　ammonia-oxidizing　bacteria　(AOB),　filamentous　bacteria　and　microorganism　community　structures　in　partial　nitrification　sludge　was　investigated　by　adding　a　pretreatment　unit　of　free　nitrous　acid　(FNA)　in　the　sequence　bath　reactor　(SBR)　for　three　days　at　1.2mg　HNO2-N/L　for　4.5.　The　results　showed　that　FNA　had　the　short-time　effect　on　AOB,　Candidatus_Microthrix　and　Cytophagaceae　of　the　dominant　filamentous　bacteria　were　also　decreased　from　5.1%　and　1.1%　to　0.78%　and　almost　invisible.　Sludge　volume　index　(SVI)　maintained　at　110mL/g　dropping　from　281mL/g,　and　the　nitrite　accumulation　rate　(NAR)　was　kept　at　around　90%,　indicating　the　partial　nitrification　was　not　undermined.　Furthermore,　High-throughput　sequencing　results　showed　that　the　diversity　and　uniformity　of　microorganism　community　decreased.　However,　the　proliferation　of　Thauera　and　Ottowia　increased　to　5.58%　and　7.82%.　Simultaneous　nitrification　and　denitrification　(SND)　had　significant　effects,　and　the　total　nitrogen　removal　rate　was　nevertheless　maintained　more　than　60%　even　only　with　partial　nitrification.　©　2019,　Editorial　Board　of　China　Environmental　Science.　All　right　reserved.