In order to investigate the effect of quickly decrease temperature on activated sludge system, with the operation of sequence batch reactor (SBR) and mean DO value at 1.5 mg/L, the sludge volume index (SVI) and pollutants removal effect with quickly decreased and return normal temperature were analyzed. The results show that the SVI increases to activate sludge bulking, and activated sludge sedimentation property worsens when the system temperature quickly decreases from 25℃ to 14℃. The SVI slightly decreases, but it cannot return to normal range when system temperature returns to 25℃. The phosphorus and COD removal effects are slightly affected by quickly decreased temperature, whlle the activated sludge nitrification is severely affected by quickly decreased temperature. with the ammonia removal efficiency decreased to 20%. The activated sludge nitrification can return to normal level when system temperature returns to normal temperature.