The invention provides a preparation method of a compound III. The preparation method is characterized in that diacyl peroxide I and a compound II containing carbon-carbon double bonds are subjected to a coupling reaction in the presence of catalysts to produce the compound III. The diacyl peroxide is introduced into a Heck reaction as an efficient and controllable alkylating agent for the first time in the method, and the method has the advantages that raw materials and catalysts are cheap, the reaction condition is mild, the operation is simple, the reaction is efficient and the like.