The downstream water level of the navigation hub is often lower than the designed lowest navigable water level during the dry season because of the factors of released clear water，sand mining and waterway dredging,etc.It will lead to problems that the water depth of the lock threshold and the submerged water depth of the water conveyance culverts are both less than the designed values and the maximum water head exceeds the designed water head.In this case,the normal operation of the ship lock may be affected.In response to the problem of a certain constructed hub,systematic analyses are conducted on the characteristics of the downstream water level changes of the dam and the characteristics of ship types crossing the dam in the past two years.The ship sinking index and surplus depth during navigation are calculated when the downstream low water level is beyond the design standard.The hydraulic characteristics of the ship lock filling and emptying process are studied under the reduced initial water depth and the increased water head conditions by mathematical model.And the corresponding hydraulic conditions of the filling and emptying system,the valve working conditions and the ship berthing conditions in the lock chamber are analyzed.On this basis,the opening modes of the valves are put forward based on the changeable downstream water level,which can optimize the hydraulic characteristic index and improve the berthing conditions of ships in the lock chamber.In addition,the engineering improvement measures of the valve system and the ship draft control parameters are given under the small submergence depth situation，so as to reduce the impact of downstream water level drop and ensure the operation of the ship lock.