In the?Code for Master Design of Shiplocks(JTJ 305—2001)，it is stipulated that the minimum water depth on sill should satisfy the requirement of?H/T≥1.6(H?is the minimum water depth on sill，T?is maximum draft of design vessel).The objectives of the stipulation are to guarantee certain speed of vessels through locks，make lock operation efficient，satisfy the demand of vessels'changeable draft during flood season and leave space for vessels upsizing.But in practice the stipulation of the code is not fully comprehended and executed.Through analysis to the application of the stipulation，it is found that the simple stipulation cannot fully meet the requirements in the engineering practice.For locks on rivers，the upstream water level is controlled by the reseroir operation rule.There are five possibilities for the lowest navigation level.They are corresponding to different downstream water levels and the minimum water depth on sill should be decided separately.