Abstract: The inflow and infiltration in the sewer system in a certain area of the Yangtze River Basin caused a significant increase of water inflow and decrease of influent concentration of the wastewater treatment plant. The project of improving the quality and efficiency of the sewer system was implemented, and the area was systematically rectified with “one plant, one policy”. During the process, based on the evaluation experience in other areas and on-site field investigation, the theoretical range of groundwater infiltration in dry days in the area was evaluated by the empirical value method, field test method and water quality characteristic factor method. The inflow and infiltration of rainwater in the area were calculated, and the theoretical inflow of the wastewater treatment plant in the area under drought and rain conditions was finally calculated. The results show that the average groundwater infiltration amount per kilometer of the wastewater pipeline in this area is 67.0 m³/d, and the rain-induced inflow amount is 71.6 m³/(km·d); the average influent of the wastewater treatment plant is 40,700 m³/d in dry days and 51,500 m³/d in rainy days. The results can provide a theoretical basis for the subsequent quality and efficiency improvement projects and the evaluation of the improvement effect, and provide a reference for the daily operation and maintenance of the drainage system in the area.