Abstract: At present, many urban wastewater treatment plants are facing the problems of low concentration and imbalance of influent pollutants. However, there are few reports on the source and quantitative analysis of leakage. In order to quantitatively analyze the influence of river water (groundwater), rainwater and pollutant degradation in pipeline on the reduction of wastewater pollutants' concentration, wastewater source of the target wastewater treatment plant was analyzed by the method of water balance trigonometry, conbined with analysis of wastewater in typical sites and the S-P degradation model. The results showed that historical water volume data combined with rainfall analysis indicated that the actual amount of primary sewage in the influent of the WWTP was only about 60.0%, and the remaining 40.0% was groundwater and river water infiltration or rainwater infiltration. The pipe network of residential quarters and municipal drainage pipe network were the main leakage sources in the sunny days, and their contribution to the reduction of inflow concentration were 20.2% and 26.4%, respectively. In addition, the pipe network of residential quarters was the main leakage source, which contributed 39.5% to the decrease of the influent concentration under 25 mm rainfall events. Therefore, attention should be paid to the completeness of residential quarters' pipe network in the process of improving the quality and efficiency of wastewater treatment.