A clay sample buried nearly 600 m in Juye mine area of Shandong province is used to have triaxial unloading test at a confining pressure of 14.0 MPa by triaxial test machine. The microstructure change of the clay sample during the entire testing process is detected by CT equipment. The CT photographs are processed and analyzed with computer image analysis method. And the characteristics of structure change of the sample during deformation and failure process under the condition of triaxial high pressure unloading test are analyzed.