Abstract: The numerical simulation technology is an effective method for optimizing the design and increasing production efficiency of improving CH4 recovery by injecting CO2 into coal seam (CO2-ECBM) engineering scheme. The paper considered the competitive adsorption, diffusion and seepage effects of CO2 and CH4 under the influence of temperature effect and effective stress, as well as the dynamic evolution characteristics of coal seam porosity and permeability. It then established the fully coupled mathematical model THM (thermo-hydro-mechanical), which described the evolution of CO2 injection coal reservoir structure-fluid migration-storage capacity and used the finite element method to solve the multi-physics full coupling problem. Then, the CO2 injection and CH4 output characteristics under different CO2 injection pressures were discussed by simulating a typical five-well pattern in No.3 coal seam in Qinshui basin. The simulation results showed that the mathematical model had a high accuracy. The simulation results confirmed that the permeability of coal seam showed a decreasing trend in the CO2-ECBM process, and the permeability near the injection well was mainly affected by the competitive adsorption of CH4 and CO2 injection. The injection of CO2 had a positive effect on improving the cumulative production of CH4, and increasing gas injection pressure can improve the efficiency of CO2 injection and CH4 output.