Aiming at solving the problem that the river bed in the dam downstream of the straight branching reach is deformed severely in the initial stage of the operation of the project, which affects navigation, this paper takes the last cascade Taoyuan Project of Yuanshui as an example to analyze the river bed evolution on the basis of collecting hydrological data, remote sensing images and topographic data during the construction period and the initial operation stage. The analysis results showed that the bed coarsening under the dam was dominated by bed load movement, and the bed coarsening began during the construction period of the project. During the same period of operation, the variation of spatial distribution of incoming flow, narrow construction diversion bundle, blocking branch and disorderly artificial sand mining had great influence on the remodeling of river bed under dam. Among them, the influence of construction diversion and artificial sand mining on river bed was very random, and difficult to predict.