Hydrophilic-activated carbon was prepared by ester hydrolysis reactions, and was characterized by surface area analysis, Fourier transform infrared spectroscopy, scanning electron microscopy energy dispersive X-ray spectroscopy, and X-ray diffraction. Hydrophilic groups that were introduced on activated carbon surface through ethyl acetate hydrolysis in an alkaline environment were more efficient than those introduced with sodium acetate. During adsorption, the hydrophilic groups on modified activated carbon surface bound with water molecules through H-bonding and increased the adsorption capacity of water vapor. The adsorption isotherms of water vapor were well fitted by the Do model. Water molecules generated larger water clusters around the functional groups at 303 and 313 K. In addition, water desorption from the samples was analyzed by thermogravimetry. Water molecules that were hydrogen-bonded to functional groups exhibited higher thermal stability than those adsorbed in the micropore of activated carbon. Besides, the process of sodium acetate formation on the surface of modified activated carbon was discussed.