Aiming at the fracture fault of the joint bolts of support pivots of tumble gate during the trial operation of ship lift in the Three Gorges, the main reason of the failure was analyzed and concluded that the pre-tightening torque 1 233 N·m was too small and the strength of the bolt A2-70-M36 was not enough. The method to improve the reliability of joint bolts of support pivots of lower head tumble gate of ship lift in the Three Gorges was proposed. Firstly, the lifting forces of hydraulic hoist cylinder and the maximum working loads of the joint bolts of support pivots were obtained under four working conditions including the gate closing, opening without water, closing and opening with water. In the working condition with water, different backwater heights of 0.8, 0.7 and 0.5 m were applied when the door was closed, and 0.8, 0.5, 0.3 m when the door was opened. Then, according to the rated output force of hoist, the backwater height was below to 0.7 m and 0.3 m in the actual closing and opening process respectively. Lastly, based on the norm of residual preload, the required pre-tightening torque was subsequently confirmed as 2 400 N·m. Strength calculation results indicated that selecting and using M36 bolt of class 10.9, the joint bolts of support pivots of tumble gate could meet the strength requirements. The operation of the tumble gate after maintenance shows that the calculation method is reasonable and can provide a technical basis for the safe operation of the lower head tumble gate of ship lift in the Three Gorges.