In this paper, physical modelling tests are performed to study the safe operation of the ship lock running as an open channel, in which the flow regime, flow velocity and hoist force at different tide levels of the upstream and downstream are modelled for the lock chamber and approach channel region. Based on the test results, the relationship between tide level difference and the maximum longitudinal velocity is established. Moreover, a method is proposed to determine the safe operation conditions of the open ship lock, in which the tide level difference between the upstream and downstream is taken as the control condition, and the flow regime, the maximum transverse flow velocity, the maximum longitudinal flow velocity and the maximum hoist force are taken as judging indexes. The tide level difference is thus determined for the safe operation of the ship lock by combining all these judging indexes. The case study for the Jiaogang ship lock shows that, a tide level difference of -0. 37 m to 0. 39 m between upstream and downstream is requested to ensure its safe operation.