The invention discloses a synthetic method of a 7-MAC intermediate, which comprises the following steps: reacting 7-ACA as raw material with MMTZ to obtain 7-TMAC; enabling the 7-TMAC and methyl sulfur bromide to undergo imidization, and then, reacting the imidization product with diphenyl diazomethane to obtain the intermediate; and enabling the intermediate and a methoxylation reagent to undergo methoxylation reaction to obtain a finished product. In the synthetic method, the 7-ACA which can be obtained easily is used as original material to synthesize the 7-TMAC, thereby reducing the production cost. In the imidization reagent of the synthetic method, methyl sulfur chloride is replaced by the methyl sulfur bromide, thereby increasing the reaction activity, improving the yield, simplifying the operation and reducing the cost.