Abstract: Since the start of operation of the Three Gorges Reservoir impoundment and power generation, the hydrological conditions in the tributary bays have undergone significant changes.Water quality in the bays is closely related to the hydrological conditions. This study took the Xiangxi Bay as the research area to explore the impact of hydrological conditions on water quality. A two-dimensional hydrodynamic water quality model of Xiangxi Baywas built using MIKE21. The changes in water quality was simulated in the bay caused by changes in water level in the main stream and upstream flow of Xiangxi Bay under various operating conditions. The results showed that an increase in the amplitude of water level variation in the main stream and an increase in the upstream flow rate of the Xiangxi Bay would significantly improve the hydrodynamic conditions of the bay. The amplitude of water level variation might increase by 1m, and the average flow rate might increase by 0.22 cm/s. The upstream flow rate might increase by 100 m3/s, and the average flow rate might increase by 0.5 cm/s; An increase in the fluctuation of the water level in the main stream would cause nutrients to migrate upstream, while an increase in the upstream flow of tributaries would accelerate the migration of nutrients to the estuary. Additionally, the enhancement of hydrological conditions in the above two cases would reduce the concentration of chlorophyll-a in the middle section of the bay; Finally, according to the distribution of chlorophyll-a concentration under comprehensive hydrological conditions, thewater area which is 0-17.1 km away from the estuary was divided into level-I water bloom sensitive area, which implied that the monitoring of this reach should be strengthened during the period of high possibility of frequent water blooms. This research results may provide theoretical support for the enhancement of hydrodynamic power, the improvement of water quality, the rational prevention and control of water bloom, and the ecological regulation in the tributaries of the Three Gorges Reservoir.