Abstract: To study the characteristics of CH₄ replacement by CO₂ under isobaric diffusion, samples of metamorphic coal, anthracite, lean coal and gas-fat coal were selected to carry out experiment of isobaric diffusion replacement under different isobaric diffusion pressure. Experimental results show that with the increase of coal metamorphism, the CH₄ and CO₂ adsorption capacity of coal is gradually enhanced, and the CO₂ adsorption capacity of coal is greater than CH₄; with the increase of diffusion pressure at the experimental point, the absolute volume and replacement rate of CO₂ for CH₄ increases, but the CH₄ replacement ratio of CO₂ injection decreases. In the range of the metamorphic degree of the experimental coal samples, the CH₄ replacement rate is negatively correlated with the coal metamorphism degree and ratio of replacement to CO₂ injection. The research results have guiding significance for the engineering technology and the theory of CH₄ replacement by injecting CO₂ into coal seam.