Eco-water demand is a key subject in ecology and hydroscience research. Under the macro background of implementing the Western Development Strategy, pursuing the ecological project of grain for green and actualizing regional sustainable development of our country, the distribution of water and soil resources, especially the deficiency of water resources has been restraining the development and utilization of resources in Western China, which should be settled urgently. In this context, it is significant to assess the amount and intensity of ecosystem eco-water demand. The authors taking Keriya River Watershed, the typical arid watershed in Western China, as an example, to assess the comprehensive eco-water demand and discuss the demand-supply equilibrium of eco-water demand by analyzing the structure characteristics of different ecosystem. The result has much significance for local government to formulate the development and utilization plan of water and soil resources, to guarantee the benign cycle and to realize the sustainable utilization of water resources, and further more to rebuild the ecological environment.