The invention provides a simple method for preparing isobutylene and alkyl styrene polymer through a solution process. The method is wider in temperature controllable range and simpler to operate. Ion liquid is adopted as a solvent of monomer, so that the method is more environment-friendly; an initiator is added one time by one time, so that polymerization conversion rate is increased and is higher than or equal to 92%, and reaction is more stable and more controllable. Molecular weight of the polymer obtained by the method is 50000-150000, molecular weight distribution is 1.8-3.0, and molar content of alkyl styrene in the polymer is higher than or equal to 3.0%.