Abstract: The seepage experiment of a sandstone sample under constant compressive stress was conducted in the laboratory using triaxial loading apparatus, seepage pressure loading apparatus and medical SOMTOM-plus CT scanner. Having analyzed the results of this experiment, the porosity formula of rock based on CT number was proposed in CT scale, and the variations of the porosity, the real flow rate in porous rock, the specific discharge, permeability and the micro-pore diameters with increasing seepage pressures were studied. The results show that the seepage parameters, such as porosity, micro-pore diameters and permeability, increase with the increase of seepage pressures. Logarithm relations between the seepage parameters and seepage pressures were proposed. The seepage pressures made the micro-pores of rock enlarged. It is shown by the relation between stress and strain of a rock in macro-scale that there is no macro-crack in the rock samples. Therefore, the deformation of rock induced by seepage pressures is elastic before macro-crack occurs. The relation between the specific discharge and seepage pressure is linear and consistent with the Darcy law in macro-scale.