Abstract: At the particle scale, the theory of thermomechanics is employed to describe the mechanism of mechanical behaviors during suction cycle paths. By means of the analysis, the plastic deformation can be decomposed into two parts, which are attributed to the structural rearrangement and change of stored plastic work, respectively. Moreover, a new constitutive model for unsaturated expansive soils is proposed, which adopts the suction-change yield surface and combines the isotropic-kinematic hardening rule to consider the mechanism of plastic deformation. The predictions are made based on the test results of suction cycles published in public. The comparisons between the measured and predicted results indicate that the proposed model can quantitatively predict the behaviors of unsaturated expansive soils under suction cycles.