The invention discloses a synthesis method of an isocoumarin derivative. The method comprises the following steps: adding 3-hexyl-4-cyan-6,7-dimethoxy isocoumarin (A), a cuprous catalyst and acetaldoxime (B) in a reaction vessel, adding a solvent N,N-dimethyl formamide (DMF), stirring and refluxing; reacting for 1 hour, cooling, filtering reaction solution in vacuum, and adding a certain amount of water in the filtrate, filtering separated solid in vacuum, drying and low-temperature crystalizing obtained coarse product so as to obtain a target product 3-hexyl-4-acylamoni-6,7-dimethoxy isocoumarin (C). The method for translating the isocoumarin into acylamino is scientific and reasonable, the step is simple, the reaction time is short, the selectivity is good, the translation rate is high, the product is easy to purify; cuprous salt is firstly used as the catalyst, and the catalysis efficiency is high in comparison with the reported cupric, in particular, the cuprous has good catalysis effect on cyan hydrolysis with large steric hindrance.