The area of the West Route Project of South-to-North Water Transfer,located in the upper reaches of the Yangtze River and the marginal areas of the Qingh ai-Tibet Plateau,has sufficient water resources but fragile eco-environment ,eco-environment water demand is the key element which determines the available quantity of water transferred because of the lower water demand for local social and economic development.Considering the effect on the eco-environ-ment ,this paper gives the available quantity of transferable water in the West Route Project of South-to-North Water Transfer for the purpose of protecting the local eco-environment and de-creasing the crisis of water resources in the western areas,by taking water demand for river channel eco-environment at downstreams of the dams as the main constraints.In order to calcu-late the eco-environment water demand,low runoff and Tennant methods are used.According to the results of risk analysis,the proposed schemes of the available quantity of water transferred have high reliability and low risk.The risk will be further reduced if multi-reservoir joint opera-tion are put to effect.