This invention refers to a kind of synthetic method of poly-armed star-like polyolefin thermoplastic elastomer, which belongs to high molecular compound of carbon-and-carbon dangling bond reaction. The synthetic method is as follows: under the ordinary pressure, low temperature and the protection of dry nitrogen gas, get the soft section of active cation polyisobutylene synthetic rubber, hard section monomer of plastic and terminator from the component solvent, initiator, coinitiator and soft section monomer, then get the 4-armed star-like block thermoplastic elastomer, whose relative molecular weight is between 180000 and 250000 and whose molecular weight distributes between 2.0 and 3.5. The advantage of the invention lies in the high molecular weight and excellent performance and refersto many thermoplastic elastomers.