The investigations in 2001-2002 revealed that Guanting Reservoir was a eutrophic lake.The density of phytoplankton was 1126.54×10⁴cells/L, with 53.4% of Cyanophyta and 32.1% of chlorophyta.Water blooms of Microcystis from July to November were noted.The content of TN and TP was separately 1.182 mg/L and 0.045 mg/L, indicating the reservoir water has come up to eutrophication.The quality ratio of TN and TP in Guangting reservoir was more than 7 and the concentration of chlorophyll-a had close relation with TP.The phosphorus was the limitative nutrient element to algal growth in the reservoir.The pollutants were mainly from point and/or non-point sources and lake sediment.To resume the function of water supplier and mitigate the staggering situation of water supply in Beijing, it is necessary to harness the pollution in the basin synthetically, i.e., to put point sources under strict control.