Although reservoir demonstrates its huge economic benefits，some ecological environment problems arise like downstream ecological water shortage，making the living environment of aquatic organisms，especially the fish suffered from varying degree of damage.Fish as the climax community in riverine ecosystem，dynamic variations of its population can reflect the whole structural information of aquatic community and water quality.This paper introduces the current researches at home and abroad from the aspects of definition and research method，and focuses on the research on natural rhythm of rivers and fish habitats.In addition，we discuss the problems in this field and present the future research trend of ecological flow.