The invention discloses a method for synthesizing a heterocyclic acetylized compound by the composite catalysis of ionic liquid and a phosphoric acid. The ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate and the phosphoric acid are taken as catalysts to perform acetylation reaction. The method comprises the following steps of: adding the phosphoric acid and the 1-butyl-3-methylimidazolium hexafluorophosphate into a reaction vessel, adding acetic anhydride, and after uniform stirring and dispersion, adding dropwise a heterocyclic compound into the reaction system, wherein the molarratio of the acetic anhydride to the heterocyclic compound is 1.1-1.3:1, and the using amount of the phosphoric acid is 5 to 15 weight percent based on the weight of the acetic anhydride; rising the temperature of the reaction system to 30 to 60 DEG C and performing reaction for 5 to 15 hours; and obtaining the target product by the conventional steps of extracting, washing, distilling and the like. The method has the advantages of not using a large amount of organic solvent in the preparation process, reducing the addition of excessive raw materials, improving the activity of the catalyst atlower temperature and reducing the generation of secondary reaction of the heterocyclic compound to improve the yield, save the cost and improve an operational environment.