The invention relates to a synthetic method of a sea natural product 8-epi-puupehedione, which belongs to the field of chemical synthesis. According to the invention, salvia sclarea aldehyde and 1-iodine-2,4,5-trialkoxy benzene are used as starting materials, salvia sclarea aldehyde and p-toluenesulfonhydrazide generate salvia sclarea hydrazone 2, the salvia sclarea hydrazone 2 and 1-iodine-2,4,5-trialkoxy benzene form a framework compound 4 of puupehedione under the catalysis of palladium in a coupling manner, the framework compound 4 is subjected to one-step hydroxyl reduction and double-bond isomerization under the action of a reducing agent to obtain an intermediate 5, the intermediate 5 is oxidized into an intermediate quinone compound 6 under the action of an oxidizing agent, and isomerization and Diels-Alder ring closure are realized for the quinone compound 6 under alkaline condition, then an intermediate 7 is obtained, and the intermediate 7 is further oxidized to obtain the final product 8-epi-puupehedione. The method has the characteristics of less reaction step, simple operation, and good products selectivity, and is suitable for industrial production.