Abstract: Without suitable interface constitutive model that can consider the influence of moistures content on the interface, there are few studies on the expansive soil-pile interface. For the purpose of carrying out the strength properties and dilatancy of expansive soil-pile interface under the influences of moisture content, the large-scale direct shear tests under different moisture contents are conducted. The test results show that the shear strength curves all exhibit the characteristics of hardening under different moisture contents and normal pressures. The normal displacement of the interface shrinks till reaching the phase transition state and then dilates, finally the interface tends to the critical state. An elastoplastic hardening constitutive model for the expansive soil-pile interface is proposed. The proposed model is compiled into the UINTER subroutine of ABAQUS through Fortran and validated by the test results. It is shown that the proposed model is capable to reproduce the mechanical behaviors of pile-soil interface under various water contents.