Abstract: The seepage experiment of a sandstone sample under constant compression was conducted in the laboratory using triaxial loading apparatus, seepage pressure loading apparatus and medical SOMTOM-PLUS CT scanner. By analyzing the results of this experiment, the porosity formula of rock represented by CT number was proposed in CT scale. Based on this porosity formula, we worked out the porosity, the real flow rate in porous rock, the specific discharge, permeability and the diameters of micro-pore with increasing seepage pressures. The results showed that the seepage parameters increase with the ascendant of seepage pressures. Logarithm relations between the seepage parameters and seepage pressures were proposed. The seepage pressures make the micro-pores within rock enlarged. This deformation within rock is elastic due to seepage pressures before macro-crack occurs. The relation between the specific discharges and seepage pressures are linear and consistent with the Darcy law in macro-scale.