In　this　study,　changes　in　the　surface　characteristics　of　microbial　aggregates　and　their　internal　mechanisms　during　Thiothrix-induced　sludge　bulking　were　explored　in　a　sequencing　batch　reactor　under　low　dissolved　oxygen　(DO)　conditions.　During　sludge　bulking　and　recovery　processes,　effects　of　the　characteristics　of　activated　sludge,　extracellular　polymeric　substances　(EPS),　and　microbial　community　on　sludge　settleability　were　investigated　comprehensively.　Significant　shifts　in　the　microbial　community　structure　of　activated　sludge　were　observed　with　changing　conditions.　Thiothrix-induced　sludge　bulking　occurred　under　low　DO　(0.3-0.5　mg/L).　During　this　stage,　sludge　volume　index　deteriorated　to　363　mL/g,　while　the　relative　abundance　of　Thiothrix　rose　from　0.01　%　to　12.69%,　making　it　the　dominant　bacterium.　On　the　contrary,　relative　abundance　of　non-filamentous　bacteria　decreased.　Furthermore,　non-filamentous　bacteria　could　not　adhere　to　filamentous　bacteria,　leading　to　structurally　unstable　activated　sludge　flocs.　When　the　DO　was　increased　to　1　mg/L,　sludge　settling　performance　recovered　and　relative　abundance　of　Thiothrix　decreased　to　1.28　%.　From　the　start　of　normal　operation　to　sludge　bulking　stage　and　then　to　recovery　process,　the　absolute　zeta　potential　of　activated　sludge　and　the　repulsive　potential　barrier　of　sludge　system　first　increased　and　then　decreased,　while　relative　hydrophobicity　showed　an　opposite　trend.　Due　to　excessive　Thiothrix　proliferation,　total　EPS　content　progressively　decreased　from　44.96　mg/gVSS　to　35.27　mg/gVSS,　with　extracellular　protein　(PN)　content　decreasing　significantly　and　extracellular　polysaccharide　content　changing　insignificantly.　Moreover,　PN　played　a　crucial　role　in　bacterial　aggregation　and　was　the　key　factor　of　activated　sludge　bulking.