In　order　to　effectively　control　the　sludge　bulking　in　oxidation　ditch　with　low　load　and　long　sludge　retention　time,　the　author　analyzed　the　process　design　parameters　and　performance　data,　and　found　out　that　the　reason　for　sludge　bulking　in　wastewater　treatment　plant　was　due　to　low　dissolved　oxygen　(DO)　concentration　caused　by　aerator　failure.　Sludge　bulking　could　be　controlled　effectively　by　reducing　the　MLSS　in　the　oxidation　ditch　according　to　the　experience　of　the　full　scale　and　laboratory　scale　continuous　flow　experiments.　The　optimum　operation　parameters　were　provided,　the　SVI　should　be　controlled　at　120-150　mL/g,　the　concentration　of　MLSS　and　DO　at　5.0　g/L　and　1.0-1.5　mg/L,　respectively.