Abstract: In the landfill, liner systems typically contain a large number of material interfaces(soil/geomembrane or geomembrane/geosynthetics). These interfaces become potential slip surfaces because of the low shear strength, which results in the instability of landfill. Based on many direct shear tests, the deformation mechanism and strain softening property of soil/geomembrane interface, the interface thickness, tangential strain and normal strain are determined. The deformation of the interface is divided into three stages: elastic stage, plastic stage and residual stage. Above all, the elastic-plastic interface constitutive model is put forward. There are only a few parameters with clear physical meanings in the model. At last, the results using this model are compared with those of the interface direct shear tests and ring simple shear tests. The comparison results show that this model is reasonable and feasible, and can be used for practical engineering.