The control measures for deterioration of aerobic activated sludge were experimented. Wastewater samples with different pH, temperature, SVI, and MLSS were investigated to detect the cause of sludge deterioration. The results show that after being operated at high S₂O₃^2- (which could lead to a pH decrease) loading in an A/O biological reactor treated for coking wastewater, though the pH was controlled at about 7.0 by the addition of NaOH, the COD removal efficiency was decreased deeply. The MLSS decreased from 3,800-4,300 mg/L to 2,020 mg/L after loading of S₂O₃^2- and NaOH in the oxic unit of A/O biological reactor for 12 days. The COD removal efficiencies and MLSS concentrations were decreased sharply when the wastewater’s temperature was over 30ºC, which indicates the bulking of activated sludge in the oxic unit of A/O biological reactor. The results suggest that it cannot inhibit the occurrence of sludge deterioration only by controlling the pH in the oxic unit of an A/O biological reactor. Although the addition of NaOH could inhibit sludge bulking, the decreased MLSS concentration would significantly reduce the COD removal efficiency. The decline of COD removal efficiency may be due to the high temperature leading to sludge death.