The urban lakes have a great difference with the natural lakes in shape, function and water circulation. The urban lakes mainly depend on manual diversion for satisfying the water need of ecological environment. The calculation method of the ecological water demand of the natural lakes cannot be used in the urban lakes. For satisfying the ecological water requirement of service function of the urban lakes, this paper sets up a calculation model for determing the water demand quantity of the urban lakes and the quality of diversion water, and taking flood control, water quality and the water level of landscape as the constraint conditions. The practicable achievement is achieved by the analysis of the ecological water demand of Zhengdong New District in Zhengzhou City.