The deformation,propagation and expansion of crack in rock interior are the main mechanical process when the rock sample comes to its crack evolution stage. It is the subject of this paper how to detect the crack width through X-ray method. The relation between rock density damage and its volumetric strain is analyzed based on the meso-mechanism of rock deformation and rupture. For brittle rock,its volumetric strain has a unique quantitative relation with its density damage. Based on the increment formula of density damage,the volumetric strain formula is derived for rock. The universal calculation formula of crack width in rock interior is derived based on the volumetric strain formula. The formula is tested and compared with relative formula. The same results are obtained from the two different formulas. Two crack widths in rock interior under different stresses are calculated. As a result the problem how to measure a real-time crack width in rock interior is solved theoretically. The crack width of sandstone is no more than the order of magnitude of 10-1mm. It is verified that the actual crack width is not the same as the crack width in its CT image. Before the maximum strength,the crack width of sandstone has a stable expansion process. When the longitudinal and lateral deformation of rock sample are considered,this result can be verified indirectly.