An A2 /O system was simulated in laboratory to investigate the effect of polymeric aluminum-iron (Al-Fe) on the performance of biological system. Chemical of polymeric Al-Fe was added in the front of the aeration tank and the effect of the dosage on the removal efficiency of TP,TN,and COD was studied. PHA content in the sludge was detected by gas chromatography analysis to estimate the effect of polymeric Al-Fe on the phosphate-accumulating bacteria. The results show that polymeric Al-Fe affects the biological phosphorus removal in A2 /O system. Under the experimental conditions,the maximum phosphorus removal is obtained at the dosage of 4 mg Al/L,TN and COD removal efficiencies are improved as well with the addition of polymeric Al-Fe. It is indicated that the total phosphorus removal in the system is determined by the combination of chemical phosphorus removal and biological phosphorus removal.