Novel PEI-impregnated UiO-66 (PEI@UiO-66) composites with enhanced CO2 adsorption capacity and CO2/CH4 selectivity were synthesized and characterized. The dynamic CO2/CH4 separation performances were evaluated in the fixed bed. CO2 working capacity and CO2/CH4 selectivity of PEI@UiO-66 at 338 K were up to 1.65 mmol/g and 111, respectively, being 12.7 and 58 times greater than those of the parent UiO-66, respectively. In the presence of water vapor in feed stream, the CO2 capacity and CO2/CH4 selectivity of PEI@UiO-66 separately reached values of 2.41 mmol/g and 251, respectively, at a relative humidity (RH) of 55% and 338 K, having increases of 48.8% and 126%, respectively, in comparison with those under dry conditions, which were higher than many MOF-based adsorbents. Density functional theory (DFT) calculations suggested that the presence of water vapor promoted CO2 adsorption, likely because of the formation of bicarbonate with a lower amine/CO2 ratio than that required under dry conditions. The multiple adsorption-regeneration tests suggested that CO2 adsorption capacity of PEI@UiO-66 can be fully recovered after regeneration.