The invention provides a preparation method for isoolefine copolymer with excellent mechanical properties and processing properties through cationic polymerization. The said method includes carrying out homopolymerization of C4-C7 isoolefine monomers or copolymerization of C4-C7 isoolefine monomers with other monomers in the hydrocarbons or halohydrocarbons diluents with composite catalyst solution. The referred composite catalyst solution is the reaction product of Lewis acid and additive, wherein the additive is selected from at least one of phenols, amides, amines, pyridines, carboxylic acids, or ketones and esters. The addition amount of Lewis acids is 10-4-10-2 mole/L based on the concentration of slurry or solution, and the molar ratio of Lewis acid to additive is at 0.01-1.0.