River hydrological condition is a natural dynamic characteristic of river ecosystem, and runoff condition is an important characteristic of hydrological condition. According to the relationship between biological population and environmental factors, hydrological indexes are selected from the runoff that reflects the hydrological conditions of rivers, and the calculation method of river ecological water demand is constructed. Five hydrological indexes, such as discharge, frequency, duration, occurrence time and change rate, are extracted from runoff to determine the river ecological water demand. The hydrological index method is used to calculate the ecological water demand of rivers in a province and compared with Tennant method. The results show that it is feasible to calculate river ecological water demand by hydrological index method.