Based on the theory of ecological hydraulic radius model (EHRM) for estimating watercourse ecological water requirement, the in-stream ecological water requirement for satisfying fish habitat in watercourse is calculated by using the example of Daofu hydrological station in Xianshuihe River at the Western Route First Stage Project of China’s South-to-North Water Transfer Scheme. Meanwhile, in order to check up practicability of this new model, the calculated result is compared with that of Tennant method. The result shows that the watercourse ecological water requirement calculated by EHRM is basic between the minimum ecological water required and condign ecological water requirement calculated by Tennant method. Because of more information of flow velocity for satisfying fish habitat taken into account, the ecological water required calculated by EHRM is more accord with the real instance of Xianshuihe River at the Western Route First Stage Project of China’s South-to-North Water Transfer Scheme. The in-stream ecological flow of every year is less than that of 40%per annual averaged flow during 1965 -1987, the range of percent is 12.0%-33.2%. If the flow is regulated by the calculated by EHRM after the Western Route First Stage Project of China’s South-to-North Water Transfer Scheme will be run, the watercourse ecosystem will be in better function state.