Abstract: Sludge reflux can increase the flocculation core of the system and achieve the purpose of strengthening flocculation. In order to explore the effect of reflux sludge under different conditions on flocculation strengthening, intelligent photometric dispersion analyzer (IPDA) was used to continuously monitor the flocculation process. By analyzing the residual turbidity and floc growth curve, the effect of sludge dosage, storage time and sludge pH were studied. Results indicated that sludge dosage had little effect on the FI value of flocculation growth, and the residual turbidity decreased to 1.3 NTU and was 38% lower than that of the conventional flocculation. The longer the sludge storage time, the smaller the FI value of floc growth, and the remaining turbidity can be maintained below 2.0 NTU when the sludge was stored within 3 days. When the sludge pH was 3, the remaining turbidity was as low as 1.2 NTU. The curve of floc growth changed unstably corresponding to the sludge treated with acid, and FI value fluctuated obviously. When reflux of single sludge, the sediment could enhance the turbidity removal ability of flocculation, and the floc particles kept bigger size. The reflux of sludge formed within 24 hours was with the largest interaction force between the floc particles, and keep the maximum activity of the sludge. The flocculation effect became worse when the pH of the sludge was too high or too low, and the most suitable pH was about 3.0.