Miyun Reservoir has become the most important source of drinking water for Beijing area.In the past 20 years,with the economic growth and development,nutrient load of Miyun Reservoir increased gradually,which will result in an increasing risk of the bloom of harmful algae.To address this issue,water samples collected from the river basins at different scales were analyzed.Total nitrogen(TN),nitrate(NO₃^--N),ammonium (NH₄⁺-N),total phosphorus(TP)and index of potassium permanganate(COD_Mn)were measured to estimate flux and occurrence of nutrients.Based on the analysis of hydrological processes for each basin,the sources of nutrients that entered into the Miyun Reservoir were estimated.The results showed that TN concentrations in almost all the samples exceeded the environmental quality standards for drinking surface water of China. Concentrations of TP in 15.9%samples were exceeded the standards.It was estimated that 40.3%of nutrients in Miyun reservoir were from point-source pollution,while the other 59.7% were from non-point source pollution.Meanwhile,50.1%of the TN,49.1%of the NO₃^--N,39.0%of the NH₄⁺-N,26.5%of the TP and 36.8%of the COD_Mn that entered into the Miyun reservoir were from point-source pollution,while 49.9%of the TN,50.9%of the NO₃^--N,61.0% of the NH₄⁺-N,73.5% of the TP and 63.2% of the COD_Mn that entered into the Miyun Reservoir were from non-point source pollution.