There is lack of systematic studies to the minimum water depth on sill needed by vessels passing through locks in China.By analyzing existing studies results，the minimum water depth on sill needed by vessels passing through locks shall be composed of draft of the vessel and additional water depth.The additional water depth is composed of squat of the vessel and the minimum clearance.Main factors influencing the squat of vessels passing through locks are blockage coefficient，speed of vessel and water depth.From application conditions of existing studies，formulas to estimate the squat through locks and limit speed of vessels are presented.The minimum clearance is used to compensate biases of the squat estimation and to take fluctuations of translatory waves into account.The main source of translatory waves is the unsteady flow from the operation of hub.In normal case the minimum clearance could be 30cm.When the influence of translatory waves will be significant on the water depth on sill，a special study shall be needed.