The CO2/CH4 competitive adsorption behaviors in bituminous coal before and after oxidation are investigated by the density functional theory. It is found that coal oxidation can decrease the adsorption energy of CO2 and CH4 on the functional groups. There exists a critical distance when CO2 and CH4 are adsorbed on the functional groups. The effects of different functional groups (-N-, -S-, -HS, -OH) of coal on adsorption characteristics are also evaluated. The interaction between CO2 and the -N- functional group is strongest among these functional groups. The adsorption performance under different CO2/CH4 ratios is examined. The displacement of CH4 will be enhanced at a higher CO2/CH4 molar ratio. Meanwhile, the grand canonical Monte Carlo simulation is carried out to analyze the effect of the coal density change caused by oxidation on CO2/CH4 competitive adsorption. The evaluation of the coal oxidation effect on CO2/CH4 competitive adsorption characteristics is helpful for an understanding of the gas adsorption mechanism in coal seams, which is beneficial to the development of CO2-enhanced coalbed methane recovery.