The study examined pollution sources in Lake Dianchi basin, China. In 2007, significant amounts of pollutants like COD, nitrogen, and phosphorus were discharged into the lake, mainly by domestic sewage. Over 20 years, while pollution loads increased, direct pollution into the lake decreased due to better sewage treatment. The problems identified include urban growth straining the lake, old drainage systems mixing clean and dirty water, inadequate sewage treatment during heavy rains, and incomplete pollution control measures. Efforts are needed to match sewage treatment capacity with demand and improve pollution control across the entire process to safeguard Lake Dianchi's water quality.