Since the 1950s, the flow diversion ratio of Ouchi River in the Jingjiang reach of middle Yangtze River has decreased yearly and even experienced long-time zero-flow in recent years with the successive construction of water conservancy projects such as the cutoffs of lower Jingjiang River, the Gezhouba Dam, and the Three Gorges Reservoir. To understand the impact of flow discharge changes in flood season on the hydrodynamics along the reach, a 2D mathematical model using MIKE 21 was established based on the measured topographic data and hydrological data in the purpose of providing reference for the practice of river regulation, dredging and river network connection engineering in the Ouchi River. Under medium and low incoming discharge, the maximum flow velocity of the river channel appeared in the Meitian Lake reach. Under medium and high incoming discharge, due to the increase in the diversion capacity of the middle branch of Ouchi River, the flow velocity in downstream Huangjinzha reduced significantly, and the minimum flow velocity concentrated in the Huangjinzha-Yinjiazhou reach. Affected by the backward slope, the hydrodynamic axis of the Yinjiazhou segment gradually shifted to the right bank, and the maximum velocity was observed at the entrance of Meitian Lake, close to the channel on the left bank. Flow diversion in the right branch was mainly controlled by incoming discharge stage and cross section area.