The synthetic CaO/Ca12Al14O33 sorbent has been regarded as an efficient CO2 sorbent during the calcium looping cycles. In this work, the CO2 adsorption performance of the synthetic CaO/Ca12Al14O33 sorbent in the presence of H2O was investigated by density functional theory (DFT) calculations. DFT was used to analyze the adsorption of CO2 and H2O on the synthetic sorbent. Both structural parameters (atomic layout and electronic configuration) and the adsorption parameters (energy, bond lengths, angles, bond population and charge transfer) of CO2 and H2O on the synthetic sorbent were determined. To confirm the feasibility of DFT analysis, the CO2 capture performance of the synthetic sorbent in the presence of steam was examined in a dual fixed bed reactor. The results indicate strong interactions between the C atom and the O atom in CaO and interactions between the H atom and the O atom in CaO. The higher adsorption energy of CO2 than H2O on pristine CaO and Ca12Al14O33-supported CaO suggests that the adsorptions of CO2 and H2O are competitive and that the adsorption of CO2 is stronger than that of H2O. The adsorption of H2O leads to the activation of adjacent O atoms of CaO and thus stronger CO2 adsorption on the O atom with H2O adsorbed. The electrons in the p orbital of the O atom near the Fermi level play important roles in CO2 adsorption by CaO. The presence of Ca12Al14O33 hinders CO2 adsorption by CaO. A low Ca(12)Al(14)O(33 )content in synthetic sorbents should be maintained to ensure high CO2 capture capacity and sintering resistance of synthetic sorbents. DFT calculations can predict the CO2 adsorption performance of CaO-based sorbents with additives under different reaction atmospheres.