Compressed air energy storage (CAES) improves the stability of renewable energy integration, with cavern sealing being a key factor for storage efficiency. To assess the feasibility of repurposing abandoned coal mine tunnels as CAES caverns, this study investigates the sealing performance of concrete air plugs. A thermal-hydraulic-mechanical (THM) model was developed using COMSOL Multiphysics software, validated against previously reliable models and semi-analytical solutions. The simulation results indicate that air pressure is the main driver of leakage, with higher pressure causing greater fracture aperture and permeability. Key factors influencing leakage include initial fracture aperture, joint roughness coefficient (JRC), injection rate, temperature, material properties of the air plug, and permeability of the surrounding rock. In contrast, higher joint compressive strength (JCS) reduces leakage. These findings provide practical guidance for the geological design and construction of CAES systems in abandoned mines, supporting the development of safe and efficient underground energy storage infrastructure.