The invention belongs to the technical field of chemical process and polymerization reactions and particularly discloses a preparation method of high-reactivity polyisobutene. The preparation method comprises the following steps: (1) mixing aluminum chloride with ether, and adding a diluent to obtain an initiator solution I; (2) liquefying isobutene and mixing with the diluent to obtain a monomer solution M; (3) enabling the monomer solution M and the initiator solution I to contact and mix in a micro-mixer so as to react in a phantastron behind the micro-mixer, and adding a terminator to the terminal of the phantastron to terminate the reaction; and (4) removing the diluent from the liquid obtained by the step (3) and drying. According to the scheme, the time of sufficient reaction is shortened to within 1min, a high-quality high-reactivity polyisobutene product of which the index is 1.6-2.0 and the terminal olefin content is over 80 and even over 90% can be obtained in a wide range of reaction temperature from -20 DEG C to 50 DEG C, and the number-average molecular weight of the product is adjustable from 500 to 5,000.