Based on a physical model with scale of 1:16，this paper studies the change rules of maximum squat to vessel’s wet cross section when ships enter and exit ship chamber and chamber water depth,ship velocity,ratio of chamber cross section.The process of ship out of chamber as control condition of chamber’s design water depth is proposed and dimensionless computing formula of vessel squat is established.With synthesized comparison of squat of ship and ship’s safe under-keel clearance under the different chamber water depth,ship chamber design water depth standard in Xiangjiaba ship lift is proposed.