This invention relates to a method for synthesizing cationic sludge dehydrator. The method comprises: adjusting the pH value of the aqueous solution of acrylamide ad acryloylethoxyl trimethyl ammonium chloride with a buffer, and initiating the polymerization reaction with a redox composite initiator. The obtained cationic sludge dehydrator has a molecular weight of 3000000-4000000, and a cation concentration of 10-30%. The method has such advantages as simple process, easy operation, mild reaction conditions, safe production, and no secondary pollution. The cationic sludge dehydrator has good dehydrating effect, and is suitable for treating wastewater and sludge from chemical and petroleum industries, especially sludge from urban sewage treatment plants.