Abstract: 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.