To　investigate　the　microbial　community　characteristics　of　sludge　bulking　caused　by　Microthrix　parvicella,　the　dynamics　of　microbial　community,　nitrifying　bacteria　and　filamentous　bacteria　were　studied　using　an　anaerobic-anoxic-oxic　(A2/O)　reactor　and　high-throughput　sequencing　technology.　Results　show　that　sludge　bulking　caused　by　M.　parvicella　was　induced　when　the　operating　temperature　decreased　from　24　℃　to　14　℃　for　21　days,　and　the　sludge　volume　index　(SVI)　value　increased　from　81　mL/　g　to　197　mL/　g.　The　average　removal　rates　of　COD　and　NH4+　-N　were　84%　and　98%,　respectively.　Sludge　bulking　changed　the　microbial　community　structure,　which　the　relative　abundance　of　Actinobacteria　increased　significantly　from　8%　to　20%　while　Proteobacteria　and　Nitrospirae　decreased.　The　abundance　of　ammonia　oxidation　bacterium　(AOB)　and　Nitrospira　decreased,　while　the　abundance　of　Nitrobacter　increased　at　low　temperature,　however,　there　was　still　a　certain　amount　of　nitrobacteria　in　the　system,　which　maintained　good　nitrification　performance.　Additionally,　the　relative　abundance　of　M.　parvicella,　Thiothrix　and　Flavobacterium　increased,　while　the　other　eight　filamentous　bacteria　decreased.　Sludge　bulking　can　be　controlled　by　increasing　the　temperature.　©　2023　Beijing　University　of　Technology.　All　rights　reserved.