Based on the integrated Geographic Information System (GIS) software package, ArcGIS9.2, a model was established to simulate an ecosystem service of avoiding the receiving waters of watershed from nonpoint source pollution and its economic value. The model included fabricating the watershed of the study reservoir. The pollutant loading coefficient and vegetation filtering coefficient of different land use/cover types were used to simulate the spatial patterns of the annual quantity of the pollutant that were prevented from entering the water environment by the vegetation in each cell followed the flow path in watershed. The economic value of the ecosystem service in this model was determined by the expected water quality standard and marginal cost of purifying pollutant. This study chose phosphorus as a pollutant surrogate and took the Ertan reservoir as an example. The results showed that the region beside the riverway in the south of watershed was the high hydraulic connectivity area. The farmland in Yanyuan county, the Xichang city and Mianning county as well as certain regions in Chengduo county in northernmost watershed, Ganzi county in central watershed were phosphorus critical source areas(CSA). In 2000, the total of phosphorus that were prevented from entering the reservoir by vegetation in watershed was 978.90 t?a-1. It occupied 81.88% of the total CSA index. The contribution ratio of the forest to this ecosystem service value was higher than those of the other contributors on a per unit area basis. The total value of this service that avoided the receiving waters from phosphorus pollution was 1370.18×104Yuan in 2000.