Cracking is one of important structural characteristics of expansive soils. The cracking damage has significant impact on the deformation, strength and stability of an expansive soil. In developing a damage model for soils, it is crucial to choose proper variable to describe the soil damage. To evaluate the applicability of several existing methods for characterizing the damage variables, a series of CT-triaxial tests was conducted on the expansive soil sampled from the site at the middle part of the South-to-North Water Transfer Project in Taocha, China. Damage variable characterization methods based on the loaded area, mean ME and variance SD of CT numbers are quantitatively examined by means of CT-triaxial shear tests under 100 kPa net confining stress and 50 kPa suction respectively. It is shown that the strength and the deviatoric stress-axial strain curves are almost the same for the soil samples with the same damage areas and different damage positions. It is appropriate for the damage variables to be defined using the damage area and the SD. However, the ME is not suited for defining the damage variable of expansive soil, though it performs well in characterizing the damage of rock. The results can provide a starting point for defining damage variable, strength and the constitutive model of expansive soil.