The injection of CO2 into deep coal seams can not only increase the recovery of CH4 but also contribute to the geological sequestration of CO2. In deep coal seams, CO2 can easily become the supercritical state when the pressure is over 7.38 MPa, and the temperature is over 31.04 degrees C. It can influence both the physical and chemical properties of coals, especially weakening the mechanical strength, which could compromise the long-term integrity and stability of the deep coal seams. Through acoustic emission experiment and triaxial compression experiment, the results show that after the treatment of supercritical CO2, the mechanical parameters of coals, including dynamic Young's modulus, static Young's modulus, rock cohesion, and peak strength, decrease significantly. It demonstrates that supercritical CO2 can reduce the mechanical strength of coals. This macroscopic phenomenon can be explained by the mechanism of the enlargement of microscopic pore spaces of coals, and this mechanism has not been studied thoroughly yet. Therefore, several microscopic quantitative experiments are comprehensively conducted, including scanning electron microscope (SEM), mercury injection porosimetry (MIP) and nuclear magnetic resonance (NMR). The results of these three tests are relatively consistent, and they show that after treatment of supercritical CO2, not only the diffusion space consisting of micropore and transitional pore but also the percolation space involving large pores and even cracks has been enlarged and expanded. This can be a significant underlying microscopic mechanism to effectively explain the weakening of the mechanical strength of coals. The pore space enlargement, together with the swelling of coal matrix, and the theoretical explanation of fracture mechanics and thermodynamic theory are all underlying mechanisms to explain the weakening behavior of coals influenced by the supercritical CO2.