Abstract: This paper discussed the general conceptions on environmental flow (e-flow). Emphasis was laid on relationship between the dynamic change of ecosystem and factors resulted by hydrological change especially by the change of flow velocity whereby to estimate e-flows. Moreover, this paper introduced a Chinese e-flow method—ecological hydraulic radius approach (EHRA)—which basis is ecological velocity and ecological hydraulic radius. EHRA estimates e-flow fully accounting for information of biota, e.g., flow velocity requirement of fish during spawning seasons, and information of rivers, such as water level, flow velocity, roughness. In addition, this paper generally described the application of EHRA in estimation of e-flow by considering pollutants degradation, requirement of fish on flow velocity, balance of river-course with erosion and sedimentation processes. What is worth noticing is that the ecological flow velocity for e-flow estimation in this paper refers to both the flow velocity suitable for biota and the flow velocity variation induced dynamic factors (e.g., ecological geomorphology, fluvial factors), which can enlarge the conception of ecohydrology and broaden its application.