Through the design of the functional group, a long-chain quaternary ammonium salts of hydrophobic monomer—C18DMMAAC was synthesized. The structure of C18DMMAAC is charactered by 1H-NMR . Hydrophobically associating copolymer HAPAM-18 was synthesized by copolymerization of C18DMMAAC and acrylic acid (AA), acrylamide (AM) in aqueous solution by free radical copolymerization . HAPAM-18 solution have a higher value of apparent viscosity under the condition of n (AM): n (AA): n (C18DMMAAC ) = 74.94:25:0.06 , AA neutralized to 95% by NaOH, monomer concentration 20wt%, initiator concentration 0.05 wt% of the monomer mass, temperature 45 ℃, and reaction time 12h. The study showed that the critical associating concentration of the copolymer solution is 2000mg / L; the polymer solution is sensitive to temperature, and the viscosity reserved rate is to 70.93% at 80 ℃; the polymer solution is pseudoplastic fluid ,but it performs shear tolerance, and viscosity reserved rate is 115% under shear rate sustaining 1 hour. Since HAPAM-18 contains lots of anionic function groups, the salt resistance of the polymer is poor.