Using sodium hydroxide as its alkali catalyzer, 3-chloro-2-hydroxypropyl trimethyl ammonium chloride as its quaternary agents, cationic starch with high degree of substitution was made by semi-dry process. The prepared cationic starch contains quaternary ammonium groups by the infrared spectrum analysis. From the scan electron microscope pictures of starch and cationic starch, cationic starch can be clearly seen to have substituted the quaternary ammonium salt groups and the structure of mutual penetration of flexible quaternary ammonium salt groups and rigid starch has formed. The tensile strength of paper improved by cationic starch with high degree of substitution is far more than that by cationic starch with low degree of substitution and poly diallyl dimethyl ammonium chloride. The scan electron microscope pictures of the surface and fracture of the paper show that the distribution of the paper fiber which added additives are apparently tight and uniform and there are many small fibers obtained on the surface of paper samples. The fibers of the paper sample which added additives have more fracture themselves and have less extraction entirely. It indicates that the force between fibers has been strengthened by cationic starch additive.