Abstract: Hydrological segmentation provides an effective approach for quantifying non-point source (NPS) pollutant load. Using this concept, a calculation formula for NPS load was established based on the segmentation of base flow. Combined with correlation and regression analyses, principles for identifying pollutantflux type and interpolating data were proposed, and the method was further improved. The NPS load from upstream rivers into the Three Gorges reservoir was studied using water quality and hydrological data from 2004 and 2005. Furthermore, acorrelative coefficient method was used to compare and explore the structural error caused by the hydrological segmentation method. The results, expressed as total pollutant flux of dissolved pollutants and particles, showed that the NPS loads of COD_Mn, TN and TP from Jialing River, Yangtze River and WuRiverin 2004 were 168.96×10⁴ ，70.30×10⁴ and 10.95×10⁴ t, respectively, and in 2005 were 228.24×10⁴, 66.64×10⁴ and 14.24×10⁴ t, respectively. Compared with point sources, NPS pollutants accounted for 60%-80% of total influx load. Regarding the spatial distribution, the Yangtze River had a dominant proportion while the Jialing and Wu Rivers only had 13.4% to 39.4%. As for the nutrient composition, the effect of nitrogen on the water was mainly through dissolved nitrogen. However, the effect of phosphorus was largely through particles. The results suggested that, although the hydrological segmentation method should be improved, at present the current hydrological segmentation method is still effective.