Abstract: A triaxial test instrument for unsaturated soils which can work with computerized tomograph was developed. The apparatus has all the function for triaxial test of unsaturated soils, moreover, it can instantaneously and quantitatively measure the interior structure of soil samples without damage. Using this device, the damage evolution for the structure of a natural expansive soil sample during unsaturated triaxial test was traced,and the relationship between the damage evolution of soil sample and its strength and deformation was also analyzed.