Abstract: Significant progresses have been made in understanding the development of liquefaction of saturated sand under shear vibration loading.But few researchers paid attention to the sand liquefaction under vertical cyclic loading.Jiaoxin railway is a high seismic intensity area after the alignment DK39+000.From the alignment DK283+550 to the alignment DK283+770,there are seismic liquefaction sand layers.It is highly possible to induce sand liquefaction by the dynamic loading of moving trains.In order to study the subgrade settlement characteristics before and after the commencement of the railway,this paper examines the concepts of sand liquefaction and the criterion of liquefaction at first.The paper further analyzes in details the settlement features of the subgrades in the sand liquefaction area.The analyses include the vertical settlement behaviors of sand layers and lateral displacement behavior.Finally,based on the test results,a constitutive model for the behavior of the sand under the condition of cyclic loading is put forward.