It is essential to research the effect of water vapor on CO2 adsorption from flue gas. Tetraethylenepentamine - impregnated hierarchical mesoporous carbon (HMC-TEPA) was fabricated as an adsorbent, and then, the adsorption mechanism and kinetics were investigated through the combination of experiments and density functional theory (DFT) calculations. The results show that both bicarbonate and carbamate are formed in the presence of water vapor, and water vapor has two effects on CO2 adsorption. On one hand, H2O has a higher affinity for TEPA, resulting in the reaction molar ratio of TEPA to CO2 decreasing from 2:1 to 1:1. In addition, increasing water vapor contents facilitates the reaction of TEPA/CO2/H2O, resulting in the CO2 adsorption capacity and TEPA efficiency increasing with the water vapor content within the range of 0 to 15 vol%. On the other hand, adsorption sites are occupied by excessive H2O (>15 vol%), and the reaction of TEPA/CO2 decreases, resulting in the CO2 adsorption capacity decreasing slightly. Under simulated flue gas conditions ((15 vol% CO2/85 vol% N2) + 15 vol% H2O), HMC-30TEPA shows a higher TEPA efficiency of 256.12 mg g−1 and a CO2 adsorption capacity of 73.48 mg g−1 at 60 °C. Three kinetic models studies indicate that the Avrami model can describe the CO2 adsorption behavior in the presence of water vapor, and the optimized adsorbent shows a higher adsorption rate. Display omitted •Water vapor had both positive and negative effects on carbon dioxide adsorption.•Both bicarbonate and carbamate were formed in the presence of water vapor.•HMC-30TEPA showed a higher TEPA efficiency of 256.12 mg g−1 under simulated flue gas.•Kinetics studies indicated the optimized adsorbent had a higher adsorption rate and a lower deactivation rate.