With the ecological hydraulic radius approach reflecting aquatic needs and channel section information, the ecological instream velocity and ecological water level in the provincial natural reserve of rare and endemic fishes in Beijiang River of Wujiang River were estimated. The results showed that hydrological elements such as low velocity, high velocity, and flow pulse featured on the ecological instream velocity process curve by ecological hydraulic radius approach, meeting the natural demands of fishes. The ecological water demand estimated by ecological instream velocity process at natural runoff could be met. Due to the cascade development on Wujiang River, the management of gate dam operation is required to guarantee the ecological water demand of the river, so as to ensure the appropriate water depth of Lishi (II) station, which is not less than the mean ecological waterdepth.