Through coagulation beaker test, research on coagulation effect of sedimentation tank slduge reflux was carried out, and the effects of PAC dosage, sludge turbidity and reflux ratio parameters on sludge reflux coagula- tion was analyzed in this study. The research results show that when the dosage of PAC was less than 30 mg/L, muddy water turbidity was from 16 to 40 NTU, and reflux ratio was from 2% to 10%, the sedimentation tank sludge water can promote the process of colloid flocculation, strengthen the coagulation effect, and improve the water quality. Besides, by way of the orthogonal test, it obtained the optimal reflux conditions: sludge water reflux ratio = 6%, PAC dosage=25 mg/L, sludge water turbidity= 30 NTU, and dosage can be saved 28.6% compared with the sludge water without reflux.