Nuclear Magnetic Resonance (NMR) imaging in combination with mechanical tests are carried out to investigate the influence of chemical solutions on porosity change, micro damage and macro mechanical property degradation of limestone samples under external stress. The NMR images and T₂ values for compression stage, micro damages emergence stage, micro damages development stage and fracture and collapse stage are obtained and analysed. The results of the corrosive influence of chemical solutions with different pH values and immersion periods on the mechanical property degradation of limestone samples are investigated. By choosing porosity as the damage variable, the micro damage of the samples during triaxial compression are calculated. It can be concluded that pH values of the chemical solutions change the porosity and micro damage evolution of the rock, which is the root reason for its mechanical properties degradation. The chemical erosion also has a significant influence on the micro crack propagation in the limestone samples under triaxial compression.