Abstract: Based on the damage theory of composite substance, an elastoplastic damage constitutive model of unsaturated undisturbed expansive soil was developed. The model can describe the complex mechanical behaviors of undisturbed expansive soil as a result of the three main characters of expansive soil, i.e. high swelling or shrinking deformation, cracks and over consolidation. In detail, the model can reflect the following behaviors of unsaturated undisturbed expansive soil, softening and bulging when sheared under low pressure, while hardening and contracting under high pressure and repeated swelling or shrinking when it was wetted or dried up. There are 23 parameters in the constitutive model which can be determined from the test results of unsaturated undisturbed expansive soil.