Cationic copolymerization of isobutylene and chlorostyrene was investigated using the Lewis acid initiator system. Titanium (IV) chloride (TiCl4) and ethylaluminum sesquichloride (AlEt1.5Cl1.5) were used as the co-initiators, and 2-chloro-2,4,4-trimethylpentane (TMPCl) and H2O were used as the main initiators. The influences of monomer feeding ratio and reaction time were studied. The reaction mechanism was proposed by studying the reaction kinetics. It was found that when p-chloromethylstyrene (p-ClMSt) was used, benzyl chloride was easily involved in the initiation reaction stage, leading to the formation of branched polymer. When p-chlorostyrene (p-ClSt) was used as a co-monomer, living copolymerization was achieved and no branching structure formed. Isobutylene, isoprene, and p-ClSt were synthesized with the AlEt1.5Cl1.5 initiator system. The high-molecular-weight halogenated ternary copolymer was successfully prepared by one-step polymerization. Vulcanization and mechanical property studies were also performed.