Abstract: The injection of CO₂ into deep unrecoverable coal seams is one of the effective ways to achieve CO₂ geological sequestration, but CO₂ will be in a supercritical state under the action of high pressure and high temperature. In order to investigate the effect of supercritical CO₂ action on coal reservoir structure, based on the self-developed supercritical CO₂ immersion experimental system, combined with acoustic emission test system and RFPA^3D numerical simulation, we studied the three coal seam thicknesses and three top and bottom lithologies of the mechanical damage characteristics and the fracture extension evolution of “Rock-Coal-Rock” (RCR) composite specimens under the action of supercritical CO₂ were investigated. The results show that: ① After the action of supercritical CO₂, the degradation of compressive strength and elastic modulus of the RCR composite gradually increases and decreases with the increase of coal thickness, while the degradation of compressive strength and elastic modulus are basically the same when the strength ratios of rock and coal are different and do not show large differences; ② The action of supercritical CO₂ will promote the plastic damage of the coal body and intensify the transformation of the RCR composite from tensile splitting damage to shear plastic damage, and the degree of plastic damage of RCR assemblage is positively correlated with both coal thickness and rock-to-coal strength ratio; ③ The supercritical CO₂ immersion promoted the RCR assemblage to enter the elastic deformation stage earlier, and the destabilization damage occurred after a more brief elastic deformation, the greater the coal thickness, the greater the influence, while the rock-to-coal strength ratio has less influence; ④ The instability potential of RCR assemblage is proportional to coal thickness and inversely proportional to rock-coal strength ratio, and the power intensity of damage is inversely proportional to coal thickness and proportional to rock-coal strength ratio; ⑤ The total energy, dissipative energy, elastic energy and surplus energy of RCR assemblage gradually decrease with the increase of coal thickness and gradually increase with the increase of rock-coal strength ratio, and the supercritical CO₂ effect will cause the elastic energy ratio of RCR assemblage specimens to decrease, the dissipative energy ratio to increase and the surplus energy ratio to decreases. Combining the above research results shows that the thicker the coal seam is, the more likely it is to be destabilized, and the higher the strength of the top and bottom rock layer is, the less likely it is to be destabilized, and the dynamic strength of the seam destabilization is inversely proportional to the coal thickness and positively proportional to the rock-to-coal strength ratio. Therefore, in a certain area of stratum that meets the premise of CO₂ injection and storage, the area of stratum with higher top and bottom rock strength and thinner coal seam thickness should be selected to store CO₂ with higher safety. The research results can provide some theoretical reference for the safety of geological storage of CO₂ injected into deep unmineable coal seams.