Abstract：Understanding the deformation and fracturing processes is a key problem for rock engineering and earthquake prediction. Experimental rock deformation studies and structural analysis of exhumed fault zones have been used to investigate the mechanism of faulting and to develop models of fault evolution. Through the study which is based on devices including the multislice spiral CT(MSCT) and real-time loading machine and seepage machine，the strain properties of sandstone under uniaxial-triaxial compressive and seepage pressure conditions are found to be different from each other. With integrated analysis of CT images and CT number，the porosity randomly distributed in the stress process is calculated. Through the variant analysis of CT numbers，it is easy to know the strain properties and cracking patterns. When the variance of CT number is increasing rapidly，the strain and cracking pattern of sandstone is brittle. If the variance of CT number is stable or declines linearly slowly，the deformation and failure modes of sandstone are plastic. The strain properties under seepage pressure are different from others. The peak strength and residual strength of a sandstone increases remarkably under seepage pressure conditions. The dry sandstone crack is brittle under uniaxial compressive condition. When the compressive conditions are with seepage pressure and triaxial compression，the crack is on plasticity. With triaxial compressive without seepage pressure，the crack pattern is between the two patterns above. So，X-ray CT is a useful method for the study of the strain properties of sandstone under uniaxial-triaxial compressive and seepage pressure conditions.
