It is necessary to understand accurately mechanical properties of coal seam induced by CO2 adsorption for the safe and successful implementation of CO2 storage. Through the triaxial compression test of coal samples (long flame coal and anthracite) under different CO2 adsorption equilibrium pressure, the change law of coal mechanical properties after CO2 adsorption is revealed. The research results show that the triaxial stress-strain curves of coal with CO2 adsorption is similar to that of coal without CO2 adsorption, however, after CO2 is adsorbed in coal, it causes the structural reorganization and expansion of the coal, which reduces the surface energy of the coal, and then leads to the obvious reduction of the compressive strength. The higher the CO2 adsorption equilibrium pressure, the greater the reduction of surface energy, and the smaller the tensile strength leading to the formation of coal cracks. CO2 adsorption causes the plastic effect of coal. This plasticization is due to the increase of segment mobility in coal polymer structure caused by CO2 adsorption, which softens the coal, and then leads to the decrease of elastic modulus of coal with the increase of CO2 adsorption equilibrium pressure. In addition, the rearrangement of polymer structure after CO2 adsorption leads to the free volume expansion of coal, the toughness of coal structure is enhanced and the elastic modulus is reduced. The degradation of mechanical properties (strength and elastic modulus) of coal caused by CO2 adsorption can be described by Langmuir curve, and the correlation coefficients are all above 0.99. In the same stress environment, the strength and elastic modulus of high rank coal are higher than that of low rank coal, it is mainly due to the existence of developed cleat system in high rank coal, which provides more conditions for CO2 adsorption. And the decrease percentage of strength and elastic modulus of high rank coal is larger than that of low rank coal after CO2 adsorption.