The water resources are the important and limited source in the world itis necessary to allocate a certain amount of ecologicalwater demand for sustainable developmental river.Based on the ecology and water environmental capacity theories, the ecological water demand of continental river system is analyzed and calculated in this paper from three aspects i.e.,the water demand for maintenance of aquatic living life survival the water demand for the maintenance of natural dilution and purification of water body and the water demand to cover the net evaporation deficiency.Itis noteworthy that this study will be beneficialfor water resources proper allocation in progress.Finally,as an example the ecological water demand in Guishui River Yongding River Basin, Beijing is calculated.