Abstract: Since the impoundment of the Three Gorges Reservoir, the hydrodynamics of the mainstreams and tributaries in the reservoir have changed significantly. It is very important to find out the reason of the density flow in the bay of the Three Gorges Reservoir. Based on the field observation data of 2018, the water flow velocity, water temperature and turbidity of Shennong Bay were analyzed. The results show that the water temperature in the Shennong Bay was not changed greatly in spring and winter, the sediment concentration was very low, and the difference of the mainstream and tributary was small, which leaded to the density flow in an irregular state. The water temperature in summer and autumn changed greatly, the sediment concentration increased greatly, and the water body most of them were submerged into the reservoir bay from the mainstream by means of medium and bottom inversion. In the flood season, the reservoir was affected by the high sediment concentration, which leaded to the increase of water density and the formation of density flow, and most of them appear in the middle and bottom of the reservoir bay. In other months, the density flow potential was formed by the temperature difference of the water body, and the temperature difference was the reverse irrigation. From June to August,the main cause of density flow was the sediment concentration of the difference between the mainstream and tributary increased, the contribution rate of sediment content to water density increased, up to 75%; the density of water in other months mainly affected by water temperature, the contribution of water temperature to water density exceeded 80% in each month, and the maximum contribution rate was 95%.