A process for preparing solvent-free aqueous polyurethane dispersion modified with acrylic grafting is divided into four steps. The first step is a reaction of a diisocyanate with a polyol to prepare a prepolymer; the second step involves diluting the prepolymer with an acrylate monomer and adding a sulfonate-based chain extender to react; the third step involves adding deionized water and a water-soluble diamine-based chain extender to obtain a solvent-free sulfonate-based aqueous polyurethane dispersion; and the fourth step involves letting the aqueous polyurethane become modified with acrylic grafting. The resulting aqueous polyurethane has excellent mechanical strength, heat resistance and water resistance.