Abstract: The low pollutants concentration of influent water from urban wastewater treatment plants is one of the key problems to be solved for improving quality and efficiency of sewage treatment. Taking a sewage treatment plant in Wuxi City as an example, based on water quality analysis and research data, the cause of low concentration of influent water was analyzed by water balance triangle method and remedial measures were put forward. The results showed that 28.9% and 9.8% of groundwater or river water infiltration and rainwater infiltration were respectively in the inflow of sewage treatment plant. Leakage of sewage pipeline network and dilution of low concentration industrial wastewater were the main reasons for low concentration of water inflow in sunny days, while the main reasons for low concentration of water inflow in rainy days were mixed rainwater in pipeline network of residential quarter, then COD and BOD₅ decreased by 65.4% and 52.9% respectively. In view of the above problems, this paper put forward four improvement measures, such as improving the drainage management system and mechanism, implementing the three pipe separation rectification for industrial enterprise drainage, refining the detection, investigation and effective repair of pipe network leakage, strengthening the acceptance and quality supervision of the pipe network, and reducing the water level of the pipe network to enhance influent concentrations of the urban wastewater treatment plant.