In this work, we have characterized four types of porous carbon material by N-2 adsorption at 77 K and CO2 adsorption at the freezing point. The results show that both the BET equation based on the N-2 adsorption isotherm and the D-A model parameters obtained from the CO2 adsorption isotherm are not applicable for analyzing the CO2 adsorption characteristics in porous carbon materials. The density functional theory (DFT) analysis results of the CO2 adsorption isotherm show that the adsorption of CO2 in a porous carbon material occurs mainly in micropores, and the micropore volume calculated by the DFT model is in good agreement with the adsorption isotherm. Therefore, the DFT model based on the CO2 adsorption isotherm is a reliable characterization method and can accurately reflect the CO2 adsorption characteristics of activated carbon.