The invention provides a preparation method of a low-molecular olefin polymer. The preparation method comprises the following steps: low-molecular olefin is subjected to a polymerization reaction under the action of a catalyst, and a product is collected; the catalyst is prepared from ionic liquid, polyphosphoric acid and organic acid salt by mixing at 30-100 DEG C for 1-12 h. The preparation method has the advantages of mild reaction conditions, high conversion rate and good selectivity, and the catalyst is easy to recover and can be recycled; the preparation method is green and pollution-free.