Ship lift is one of the two navigation structures of the Three Gorges project，which mainly provides fast passage for ships. While ships entering and leaving the ship chamber，the longitudinal overturn moment caused by the change of the fluctuation of the water surface will be undertaken by the docking and locking system. If the change exceeds the load limitation of the docking and locking system，it may threat the docking safety of the ship chamber. A physical model scaled 1∶12 of TGP ship lift chamber and navigation channel is established. Through hydraulic model experiment of 3 500 t bulk cargo ships entering and leaving the chamber，the relationships among ship draught，ship speed，water surface fluctuation and the force on the docking and locking system are analyzed. In order to protect the docking and locking system，it is suggested that the maxium draught of 3 500 t bulk cargo ship is no more than 2.70 m when its speed of passing through ship chamber is 0.5 m/s.