Abstract: To investigate the meso-damage evolution characteristic of unloading rock mass,unloading confining pressure tests on marble specimens were first conducted under initial confining pressures of 10,20 and 30 MPa. Then nuclear magnetic resonance tests were performed on the unloaded specimens. The stress-strain curves,transverse relaxation time T₂ distribution,rock porosity,and nuclear magnetic resonance imaging pictures of the unloaded specimens were obtained. It is found that the deformation of the unloaded specimens changes from elastic to plastic with the increase of unloading confining pressure ratio. As the unloading confining pressure ratio increases,the size of small microcracks increases,and the number and size of large microcracks also increase. When the unloading confining pressure ratio is less than 90%,the damage of rock mass is mainly caused by the increase in number of microcracks.But when the unloading confining pressure ratio exceeds 90%,the damage of rock mass is caused by the sharp increase in number and size of microcracks. The rock porosity increases with the increase of unloading confining pressure ratio,and the growth rate becomes greater. The development processes of the number,size and structure of microcracks in the rock specimens under different initial confining pressures and different unloading confining pressure ratios can be directly visualized by nuclear magnetic resonance imaging pictures.