Abstract: To study the contact problem between unsaturated soil and structure, based on the state-dependent concept and the critical state theory, by considering the influences of suction on the yield function, flow rule and hardening law and so on, an elastoplastic interface model is established with the net normal stress and the suction serving as stress state variables. The reliability of the model is demonstrated through the shearing tests on sand-steel and sand-geotextile interface. The results show that the proposed model can describe the mechanical behaviors of sand-structure interfaces under different initial states, and can predict the variations of shear stress, normal displacement and stress path of interface subjected to different boundary conditions (i.e., constant normal load, constant normal stiffness and constant volume condition) as well. Thereafter, the shearing test results of unsaturated silt-steel and completely decomposed granite (CDG)-cement interface under different suctions are predicted, finding that with the increase of suction, the shear strength of interfaces increases, the strain softening and dilatancy behaviors become more significant, and the shear displacement corresponding to phase transformation point representing the transition from contraction to dilation decreases. Compared to those of the existing models, the parameters of the proposed model are easier to be calibrated, the calculated results are closer to the measured data, and the decreasing trend of the shear displacement corresponding to the peak strength with suction can be reflected, indicating that the model here captures the effects of suction on the peak strength, critical state and hardening behaviors of the interface better.