The current criterion takes the navigable flow condition of the entrance area of the ship lock approach channel as the discriminative standard of the ship accessing safety.Through investigation and research,it is not comprehensive to consider only the navigation flow condition in the entrance area,but also the movement characteristics of the ship in the area.Based on the momentum conservation law,considering the effect of the flow on the ship,we analyze the characteristics of the movement of a ship for the maintenance of safety,research the relationship of the flow conditions and ship navigation conditions by their model tests,and propose a method to determine the safe navigation conditions of ships in the entrance area of the lock based on momentum conservation law.The results show that it is reasonable and feasible to judge the navigation safety of the ship in the port area based on the minimum critical velocity calculated from the test results of flow conditions.