Abstract: Analyzing the spatiotemporal distribution of total nitrogen (TN) and total phosphorus (TP) and identifying their sources are critical for effective plateau lake management. This study uses Yilong Lake, an eutrophic plateau lake, as a case, employing an enhanced output coefficient model to quantify the diffuse sources of TN and TP from 1990 to 2020. Point sources of TN and TP were subsequently calculated using the pollutant discharge coefficient method. Using GIS spatial analysis, the spatial distribution characteristics of TN and TP pollution at the sub-basin scale were analyzed. The results indicate that (1) TN and TP pollution in the Yilong Lake watershed increased initially, peaking at 398.79 and 27.90 tons, respectively, in 2010, before declining through 2020. (2) The primary sources of TN and TP pollution were fertilizer application, rural domestic activities, and livestock, contributing 28%-50%, 20%-40%, and 7%-22%, respectively. (3) The Chengbeihe sub-basin experienced the highest TN and TP pollution, reaching 1 750 kg·km^-2 in 2010 and 140 kg·km^-2 in 2005, respectively. Fertilizer application was the predominant source of TN and TP pollution in the Nanansanliu area and the Chenghe sub-basin. TN and TP pollution from rural domestic activities primarily originated in the Chengnanhe and Chengbeihe sub-basins. The Beiansanliu area and Longganhe sub-basin were the main sources of livestock production. The findings of this study not only provide data to support eutrophication control in Yilong Lake but also offer valuable insights for water environmental protection in other plateau lakes.