Taking the downstream of the dam of hydropower station as the research object, by analyzing the natural day flow characteristics, a diagram of river ecological flow characteristics was drawn based on the monthly mean flow and flow of typical characteristics, being the standard of ecological flow of reservoir water release. Based on the diagram of river ecological flow characteristics, the deviation rate of 7 day flow, the guarantee rate of 7 day ecological water demand, the monthly average suitability degree of ecological water, and the comprehensive index of ecological water demand based on these three indexes were set up to judge the satisfaction level of ecological flows; and the evaluation basis and evaluation method of each index were given. The results show that the diagram of river ecological flow characteristics conforms to the basic characteristics of river natural flow of each period, and it can be used as a basis for guiding the reservoir ecological discharge; and the evaluation method of ecological flows set up can reflect the changes of discharge flow and natural flow, and it can evaluate the satisfaction degree of river ecological flow.