We report a preparation of water-soluble cationic starch and its application as an oil-displacing agent in chemical enhanced oil recovery. The singular sand-pack core-flood tests provide experimental evidence that the injection of the modified starch after the conventional water flooding can significantly enhance oil recovery and effectively reduce water-cut. To discuss the proposed oil-displacement mechanism of this cationic starch solution, its static and dynamic adsorption properties were also presented in this paper.