The invention discloses a preparation method of hydrophobic association type cationic polyacrylamide. The preparation method specifically comprises the following steps of under the condition of existence of a nonionic surfactant, adding a certain ratio of acrylamide monomer, cation monomer, low molecular weight anionic polymer and deionized water into a reaction device, and completely and uniformly stirring, so as to form a micelle copolymerizing system; adding a small amount of solubilizer and photo-sensitive initiator into the micelle copolymerizing system, and uniformly stirring; after theoxygen in the device is completely exhausted, sealing the device, placing into an ultraviolet reaction device, and performing photo-initiating micelle copolymerizing and reaction, so as to obtain a hydrophobic association type cationic polyacrylamide gel with a block structure; finally, purifying the gel, refining, drying and granulating, so as to obtain a final product. The prepared hydrophobic association type cationic polyacrylamide has the advantages of hydrophobic association property, centralized distribution of cationic groups, strong electron neutralizing ability, high molecular weightand the like, has good flocculation property, and can be widely applied to multiple types of sewage treatment and sludge dewatering technology.