A series of hydrophobically associating cationic copolymers, poly(AM-DMDAAC-BA), were synthesized with acrylamide (AM), dimethyldiallyammonium chloride (DMDAAC) and butylacrylate (BA) by the micellar free radical copolymerization technique. Copolymerization was initiated by a complex initiation system, which was composed of a redox initiator (K2S2O8-Na2S2O3) and thermal decomposition initiator azobisisobutyronitrile (AIBN). The effects of synthesis conditions, such as various initiation systems, initiator concentration, monomer ratio, surfactant concentration and urea concentration on properties of polymers were studied. The results showed that conversion and molecular weight of polymer were increased with the use of the initiation system. The optimum conditions of copolymerization were as follows: the molar ratio of AM, DMDAAC and BA was 68.6∶29.4∶ 2.0, respectively and the percentages of initiator, surfactant and urea by weight in the monomers were 0.15%, 2.0% and 2.5%, respectively. Infrared spectrogram of the copolymers proved the formation of hydrophobically associating cationic copolymers. The results of coagulation performance experiments indicated that the oil removal rate was significantly improved by using P(AM-DMDAAC-BA).