Using the rock mechanics test system RMT150 and high-resolution CT medical machine, the siltstone specimen CT scan tests under a constant confining pressure and different axial loadings were carried out and clear CT images of rock section were obtained. On the basis of the CT number’s principle, a quantitative analysis on damage evolution and damage degree was performed. The micro-damage process of rock was reflected by the sample’s 3D CT image reconstruction. The test study results showed as follows: The test rock specimen experiences three stages including compaction, expansion and destruction. The compression damage evolution of the rock is a long-time accumulation process and the crack generation and its expansion process manifest a sudden change， and there exists a ‘density rebound’ phenomenon in the region away from fracture plane.