The invention relates to a synthetic method of tung oil-based dimer acid, which comprises the following steps: in the presence of a polymerization inhibitor, a linoleic acid dispersing agent and a catalyst, conducting stirring while nitrogen purging, conducting heating to 180-230 DEG C, preferably 190-210 DEG C, beginning to feed tung oil acid in a flowing manner, and maintaining reaction for a period of time after feeding is finished to obtain a dimer acid product. The dimer acid is synthesized by the method disclosed by the invention, and the molecular formula of the dimer acid is C34H58(COOH)2. According to the method, eleostearic acid is used as the raw material to synthesize the dimer acid, a proper catalyst is used, the reaction temperature is reduced, and the method has the advantages of relatively mild reaction conditions, high dimer acid yield, small trimer acid generation amount and the like.