The kinetics and mechanism of low-concentration CO2 adsorption on a solid-supported polyethylenimine (PEI) adsorbent in a humid confined space were investigated, where the content of CO2 was about 0.4 % (v). A flow system, equipped with a dew point meter and IR gas analyzer connected in series for online measurement, was developed and the co-adsorption performance of CO2 and H2O was analyzed. It was found that the H2O adsorption capacity and activation energy were not significantly changed when 0.4 % (v) CO2 was applied in the co-adsorption process, indicating that the effect of dilute CO2 could be neglected. However, the CO2 adsorption capacity increased with the assistance of H2O, and then it decreased slightly when the H2O content in air flow was above 1.56 % (v). This implied that the adsorption activation energy of CO2 could be significantly reduced due to the existence of H2O in air flow. The direct formation of bicarbonate on the supported PEI adsorbent was favoured for the co-adsorption process of CO2 and H2O.