Abstract: In this paper,we firstly,introduce the present research situations about ecological water deman,develop and analyze theoret ically,a new method for calculating the ecological water demand,combining the ecological economy theory based on sustainable development.According to the ecological value theory,the ecological values for different ecological water demand are confirmed.The relationship between ecological value and the economic value of water resources is used to confirm the ecological water demand.A case is used to show the concrete steps of this method The ecological water demand confirmed by this method fit with actual conditions,and can be used to study the ecological water demand theory during environment construction.In this method,the ecological system and the economy system are combined together by water resources.And this method has significance for studying the water resources utilization in the condition of sustainable development.