Abstract: Longdun Reservoir watershed was chosen as a case for study to predict nitrogen and phosphorus pollution load using GIS(geographic information systems) and USLE(universal soil loss equation).The watershed was divided into subwatersheds using the subwatershed division module of SWAT model. And the non-point source pollution of the subwatersheds were evaluated,using the indices of equal-standard pollution load and equal-standard pollution intensity. In the end,nitrogen and phosphorous pollution loads were graded through cluster analysis and critical source regions were defined. Results of the study show that in the studied watershed,the pollution load and intensity of total phosphorus(TP) is124. 25 t and 4. 95 t·km^-2,respectively; and the pollution load and intensity of total nitrogen(TN) is 274. 87 t and10. 96 t·km^-2,respectively. The equal-standard pollution load of the whole watershed is 2. 76 × 10⁹m³,of which 2. 75× 10⁸m³ is equal-standard TN pollution load accounting for 9. 96% and 2. 48 × 10⁹m³,is equal-standard TP pollution load,accounting for 90. 04%,indicating that TP is the dominant non-point source pollutant of the watershed. TN and TP pollution loads and pollution load per unit area varies with type of land use. Upland is the highest in both and hence the major source of the non-point source pollution in the Longdun Reservoir watershed.