Abstract: To address issues in CO2 capture and in-situ utilization such as the separation of adsorption and catalytic sites in existing materials and harsh reaction conditions, this study selectively quaternized hypercrosslinked polyethyleneimine (MPEI) via an ammonium iodide solution impregnation method, yielding an amine–ammonium bifunctional material (MPEI-I) featuring synergistic amine-based adsorption sites and quaternary ammonium catalytic sites. The effects of water content and reaction temperature on CO2 adsorption and in situ carbonylation were investigated. Experimental results show that the CO2 adsorption capacity of MPEI-I under humid conditions is 1.17 mmol/g, which is 31% higher than that under dry condition. Under conditions of 70 °C, 1.0 MPa CO2, and 20 μL H2O, the yield of butylene carbonate from the CO2 cycloaddition reaction was nearly 12 folds higher than that under anhydrous conditions, demonstrating the enhancement of water-promoted amine adsorption process to the CO2 cycloaddition. Increasing the water content to 100 μL did not cause attenuation in butylene carbonate yield, indicating excellent water resistance of the material. The improvement of water to the reaction is attributed to the formation of a hydrated carbamate intermediate. Apart from increasing the CO2 adsorption capacity, the hydrated carbamate intermediate exhibits a lower decomposition temperature than the carbamate (dry CO2 adsorption intermediate), thereby achieving efficient coupling between the CO2 adsorption and the CO2 cycloaddition. In summary, a bifunctional material MPEI-I with an "adsorption–catalysis" synergistic effect has been successfully constructed through a selective quaternization strategy, enabling efficient capture and in situ conversion of humid CO2 under mild conditions.