The middle and lower reaches of Fenhe River is in the arid and semi-arid area of the Loess Plateau of China and shortage of water resources and eco-environmental issues are very prominent.In views of the eco-environmental problems in development and utilization of water resources,this paper aims to construct a calculation model of ecological water demands in the middle and lower reaches of Fenhe River.Because the demands change with time and river sections,the middle and lower reaches of Fenhe River are divided into five sections according to hydrological stations for the calculation of ecological water demands with different hydrological frequencies.Ecological water demands are classified as river ecology surface evaporation,seepage demand,self-purification demand,sediment transport demand,and base flow demand.According to the relationship among them,total eco-environmental water demand is defined.Annual ecological water demands with the 4 hydrologic frequencies of 25%,50%,70%,and 95% are 617.43,377.94,236.93,and 158.56 million m³;the percentages over annual ecological water demand by channels are about 55.43%,61.97%,61.27%,and 88.15%;and the percentages over total flow averaged over years are 73.68%,45.10 %,28.27%,and 18.92%,respectively.It is necessary to take effective measures to control water resources for the sustainable management of Fenhe River basin and make use of water resources in different periods and reaches.