Based on the integrated Geographic Information System (GIS) software package, ArcGIS9.2, a model was established to simulate an ecosystem service of avoided\|reservoir\|sedimentation and its economic value. Relevant data of the year 2000 were used in Ertan reservoir by this model. The quantity of avoided\|reservoir\|sedimentation and the economic value of this ecosystem service for the reservoir′s designed operating life(100a) were simulated under ten different land use/cover scenarios as well. The results showed that in 2000, the total quantity of soil retention in the watershed of this reservoir was 12.1×108t?a-1. The region with higher soil retention was near the main and branch streams of the Yalong River; The sediment delivery ratio that was near to the stream and the Ertan reservoir was higher. The region with a higher quantity of sediment retention was around the reservoir; For the reservoir′s designed operating life(100a), the total value of the avoided Ertan reservoir sedimentation service in the watershed was 2.75 billion Yuan. The influence that different land use/cover scenarios′ pattern had on the ability of sediment retention was different. The simulated results of sediment retention accorded with the regulation that was: forestland>grassland>wasteland>farmland. The farmland′s ability of sediment retention was the weakest. It was the most important source of sediment yield in the watershed of Ertan reservoir.