The invention relates to a preparation method of entecavir. The method comprises the steps: by adopting butanedial as a raw material, firstly synthesizing optically pure double-loop olefine aldehyde in the presence of a catalytic amount (R)-proline and dibenzylamine trifluoroacetate, protecting hydroxyl by using methyl, and carrying out decarbonylation by utilizing chlorotris(triphenylphosphine)-rhodium as a catalyst to obtain (3aS, 6aR)-2-methoxyl-3,3a-6,6a-tetrahydro-2H-cyclopentano [b] furan; carrying out Prins reaction in the presence of sulfuric acid to obtain a reduzate of Corey lactone, carrying out deprotection on all hydroxyls by using TBS, selectively removing a 2-bit protecting group and carrying out decarboxylic reaction in the presence of lead acetate to obtain a methylene compound; and finally introducing guanine by adopting a conventional method and carrying out deprotection to obtain the entecavir. By adopting butanedial as an initial raw material, the method has less than ten steps, and is mild in reaction condition, available in raw material and reagent, simple in operation, moderate in yield and suitable for industrial production.