Abstract: The hydrological regime of the mainstream in the middle and lower reaches of the Yangtze River and the lakes connected to the Yangtze River have been changed remarkably by the operation of the Three Gorges Reservoir. The impacts of the Three Gorges Reservoir operation on the hydrological process of the Yangtze River and Poyang Lake from 2009 to 2014 were evaluated based on one and two-dimensional hydrodynamic models and the observed hydrological data. The operation of the Three Gorges Reservoir had a relatively large impact on the water exchange process from May to October in Hukou Station, and back flow was decreased from July to October. The flow of the mainstream in the middle reaches of the Yangtze River was reduced during the water storage period of the Three Gorges Reservoir, especially in October, and the average water level of Hukou Station was decreased by 1.09m. The maximum influenced reach can extend to Kangshan in the water storage period. During the water replenishment period, the flow of the mainstream in the middle reaches of the Yangtze River increased significantly from January to March, and the average water level at Hukou station increased by 0.28~0.56m. The maximum influenced reach can only extend to Duchang. In the flood season, the highest water level at Hukou Station raised 0.34m from May to June, and decreased from July to August. The research results can provide reference for the comprehensive management and protection of Poyang Lake.