Abstract: Based on experimental observations and cause analysis of formation,an intrinsic relationship was revealed between the large post-liquefaction shear deformation of saturated sand under undrained cyclic loading and three types of volumetric strain components(i.e.,a reversible component due to dilatancy,an irreversible component due to dilatancy and a component due to change in mean effective stress).It was found that the development of the large post-liquefaction shear deformation is governed by coupling variation of the three volumetric strain components and is accompanied with three physical states of soil particles(i.e.,the frictional contact state,the critical contact state and the suspension state) that appear alternately.The above new knowledge provides a rational explanation why unstable flow slides and large reconsolidation deformation may take place after the initial liquefaction and also a rational approach to the establishment of an elasto-plastic constitutive model used to describe the large post-liquefaction deformation.