Abstract: In order to study the effect of moisture on CO₂ displacing methane in coal, a set of isothermal adsorption experiments of CO₂, CH₄ and their mixed gases were carried, and the adsorption potential energy of coal adsorbed water was calculated by the optimized BET model. In this experiment, two highly metamorphic coal samples of fat coal and anthracite coal with different moisture contents were prepared by the water vapor adsorption partition method. The results show that the adsorption capacity of coal to water vapor can be fitted by the optimized BET model. The first-order and second-order adsorption potential energies of anthracite for water are E₁=44.20 kJ/mol and E₂=42.27 kJ/mol, respectively, which are larger than the adsorption potential energies of coal for CH₄ and CO₂, indicating that the water has a displacement effect on gas in coal. The adsorption capacity of CO₂ is greater than that of CH₄ under the same conditions. The ability of coal to adsorb CO₂ and CH₄ gradually decreases with the increase of water content, and the displacement effect of H₂O on CH₄ in coal is significantly enhanced. The displacement of CH₄ in coal by CO₂ decreases with the increase of water content, and the displacement rate is basically unchanged. Moisture plays an inhibitory role in CO₂ displacement.