Abstract: Based on the Kachanov’s continuous damage variable principle and the Fredlund’s effective stress theory for unsaturated soil, a strength characterization method for structure-damaged expansive soil is proposed. The soil taken from South-to-North Water Transfer Project in Nanyang is reconstituted. 15 specimens are divided into three groups with different cylindrical-pore damages. Whereafter, triaxial compression tests are conducted by using a modified multifunction unsaturated soil triaxial apparatus under controlled net confining pressures of 50, 100 and 150 kPa respectively. The influences of the initial pore-damage primitives and fissure evolution under triaxial loading on the mechanical behaviors of expansive soil are analyzed quantitatively. The results indicate that the mechanical characterization of unsaturated soil associated with structure and damage can describe the mechanical behaviors of soil commendably. Cylindrical-pore damage has no consistent weakening or strengthening effect on the strength, while the fissure morphology and evolution forms of soil play a significant role in the structure and strength. The development of fissures can be restrained by the net confining pressure at a certain extent, and the strength is kept through the fissure surface occlusion. Particularly, its mechanical behaviors are characterized by two stages of fissure development and residual strength, and four damage types of linear-hardening, elastic-plastic, brittle-plastic and linear-softening failure modes. The study may provide new references for revealing and predicting the failure mechanism of expansive soil slopes.