The present invention relates to an aminomethyl polystyrene resin wherein aminomethyl group is grafted only on the surface of polystyrene resin, and a preparation process for the same. More particularly, the present invention is directed to a process for preparing aminomethyl polystyrene resin wherein aminomethyl group is grafted only on the surface of polystyrene resin, which comprises a step for grafting amidomethyl group to surface of polystyrene resin and a step for hydrolysis of the amidomethyl group grafted on the surface of polystyrene resin.