Abstract: Considering the excellent properties of supercritical CO2 has economic and technological advantages on the development of unconventional gas resources. This study aims to investigate the quantitative effects of CO2 and supercritical CO2 on the structure and mechanical properties of sandstone. Hence, acoustic emission (AE) and low-field nuclear magnetic resonance tests are carried out to analyse the damage of sandstone by soaking with CO2 and supercritical CO2 for 15 days. The results show that uniaxial strengths of samples under these two conditions decrease, and the results under supercritical CO2 soaking condition are even lower. In addition, samples subjected to supercritical CO2 soaking produce a large number of AE signals in the elastic stage. The growth points of accumulated AE curve (the cut-off between the crack closure and crack initiation) for all samples appeare in smaller strain stages. The AE energy probability densities P(E) of all samples satisfy the power law well, CO2 and supercritical CO2 soaking conditions do not change the scale invariance of AE energy for sandstone. The probability density curve is more inclined under the supercritical CO2 soaking condition, which reflects an increase in the proportion of small AE energy signals. The nuclear magnetic T2 waveforms of samples soaked with supercritical CO2 represent more developed porosity, internal microstructures and higher proton NMR signals.