Flow regime change is the leading driving force in the evolution of lacustrine ecological systems. According to the planned water level regulation scheme of the Poyang Lake hydraulic project (PLHP), this research discusses the possible impacts of PLHP on the hydrology and hydrodynamics of Poyang Lake (China) using the method of scenario simulation. The results show that a controlled water level of 11 m would effectively raise the average water level during the low-flow period of the lake in a typical dry year, normal year, and wet year, leading to the decrease of flow velocity in the lake. Those changes were more obvious in regions to the north of Tangyin, especially in the waterway of Poyang Lake into the Yangtze River. The flow field to the north of Tangyin under the scenario with the PLHP was apparently different from that under the scenario without PLHP. Furthermore, the PLHP had a significant impact on the water exchange period of the lake due to the increase in the water storage capacity and the reduction of flow velocity. These results can provide support for further quantitative analysis of water quality and the evolution of ecological systems in the lake affected by the PLHP.