The presence of water vapor complicates the separation and recovery of CO2 from flue gas. Activated carbon is widely regarded as a promising adsorbent for CO2 capture because of its hydrophobicity. However, complete hydrophobicity is impossible, especially at high humidity levels. The effects of water vapor on CO2 capture systems using activated carbon must be further investigated. An adsorption isotherm expression of water vapor on activated carbon named Langmuir + arc-tangent model is presented in this paper, based on which a three-component adsorption equilibrium expression of H2O/CO2/N-2 on activated carbon and a complete mathematical model for CO2 adsorption and recovery from wet flue gas by using activated carbon-fixed bed are established. Vacuum swing adsorption (VSA) process for CO2 recovery from wet flue gas under different inlet relative humidity and temperatures are studied, and the role of water vapor on the adsorption and desorption processes of CO2 on activated carbon is revealed. Suggestions that are favorable for CO2 capturing from wet flue gas by using activated carbon under certain conditions are proposed.