Construction and operation of reservoirs, dams, channels and other hydraulic projects altered the flow regime of rivers and their morphology evolution, and caused other problems including degradation in the ecological functions and reduction in the species diversity of rivers. This paper reviews the research on river environmental flow and its development in recent three decades, points out its three developing stages, and examines the characteristic of each stage. Facing with changing environments, further studies of river environmental flow should pay more attention to the integrity of ecosystem, modeling its mechanism, and the applicability in practice of ecological operation. In such studies, climate change, spatial heterogeneity, and correlation between ecological and social systems would become the frontiers and major focuses.