Abstract: To investigate the effect of water content in flue gas and moisture in coal on the greenhouse gas CO₂ storage and coal spontaneous combustion when injecting power plant flue gas into goaf, the dry and wet coal structural models were established. The adsorption behavior of flue gas mixtures of CO₂/O₂/N₂/H₂O with different water contents by dry coal and wet coal with various moisture contents was simulated using Grand Canonical Monte Carlo method. The result shows that the competitive adsorption ability and capacity of CO₂ in flue gas are the strongest, while the physical adsorption amount of O₂ is the least. The H₂O content in flue gas has little effect on the adsorption amount of CO₂, N₂ and O₂. So the power plant flue gas can be directly injected into the goaf without drying. As the water content in the coal increases, the pore space is occupied by water, the Van der Waals force decreases, and the hydrogen bonding between H₂O-H₂O increases to provide additional adsorption sites. However, because the isosteric heat of H₂O increases and the adsorption sites move toward a lower interaction region where the adsorption is stronger, a large amounts of water is adsorbed to form water clusters and compete the adsorption sites and space with CO₂, O₂, N₂, leading to a decrease in gas adsorption amount by more than 50%. Therefore, the water content in goaf coal should be fully taken into account when the flue gas is injected.