In response to the problem that the diversion power station changes the hydro-hydraulic characteristics, topography and other natural conditions of the river and forms a dewatered section between the power station and the diversion dam, which damages the ecological environment of the river, the downstream of Zhenquan Dam of Houxi River, a secondary tributary of Yangtze River, was selected as the study section. Through field research and data collection, the local indigenous and key fish, Schizothorax prenanti, was taken as the indicator species. Considering the ecological process of its whole life cycle, the suitability curves of water depth and flow velocity during spawning period are summarized. On the basis of identifying the distribution of current geomorphic units in the study river section, considering the threshold values of water depth and velocity and substrate characteristics, establishing the relationship between water depth, velocity and hydraulic units, identifying the area change of feeding grounds and wintering grounds. Combined with the Tennant method, the optimal, appropriate and minimum ecological flow of the dewatered section is determined in phases.