In order to analyze the characteristics of the unsteady flow of the downstream approach channel of the Three Gorges ship lift on the operation of lower head tumble gate，we use the Flow-3D to establish a three-dimensional model of the tumble gate operation region and the ship chamber of the Three Gorges ship lift，calculate and analyze the influence factors of different downstream water levels and different fluctuation amplitude flow on the lower head tumble gate by the water flow fluctuation load generalizability theory，analyze the relationship between the calculated moment of the downstream unsteady flow acting on the tumble gate and the measured moment of the tumble gate closing process，and propose the safe operation strategy of the tumble gate to reduce the security risk of the lower head tumble gate due to the unsteady flow of downstream. The results show that the small water surface fluctuation in the downstream approach channel will cause great water surface fluctuation when it transmits to the lower head tumble gate area of the Three Gorges ship lift，but the downstream water level of the hub has little effect on the tumble gate operation.