A process for synthesizing thermoplastic polyolefine elastomer includes preparing the soft segment of synthetic active cationic polyisobutylene rubber from mixed solvent, trigger, cotrigger, nucleophile and soft segment monomer at low temp and normal pressure under the protection of dry nitrogen gas, and adding blocking agent, plastic hard segment monomer and end stopper to obtain thermoplastic block elastomer with a molecular weight range of 7000-95000 and a molecular weight distribution of 1.26-1.44. Its advantages are narrow molecular weight distribution and excellent properties.