The adjustment of runoff by a hydro-junction after its construction destroys the inter-annual consistency among upstream and downstream water level samples，consequently complicating the determination of the designed navigable water level.Focusing on the problem of selecting the designed lowest navigable water level for a ship lock expansion project of a cascade hydro-junction，this paper discusses the double-line ship lock of the Guoyang Hydro-junction as an example.Specifically，it statistically analyzes the inconsistent water level sequence and selects the representative hydrological sequence for the calculation of the guaranteed water level.Then，the paper compares the guaranteed water level with the dead water level and the lowest navigable water level of the hydro-junction and discusses the rationality of the designed navigable water level selected according to factors such as project investment.The results show that using the dead water levels of the Guoyang and Mengcheng hydro-junctions as the designed lowest navigable water levels of the upstream and downstream can not only meet the navigation guarantee rate with favorable economy but also ensure the water level connection between the cascade hydro-junctions and the navigation safety of ships.