The invention provides a method for preparing 2, 4, 5-trifluoro-phenylacetic-acid. The method adopts 1, 2, 4-trifluoro-
benzene to react with polyformaldehyde and chlorinating agent to obtain 2, 4, 5-trifluoro-
benzyl chloride, and adopts the 2, 4, 5-trifluoro-
benzyl chloride to react with cyaniding agent in
ionic liquid to obtain 2, 4, 5-trifluoro-
benzyl cyanide, and then the 2, 4, 5-trifluoro-
benzyl cyanide is hydrolyzed in acid or alkalic conditions to obtain the 2, 4, 5-trifluoro-phenylacetic-acid. The preparation method for preparing 2, 4, 5-trifluoro-phenylacetic-acid requires cheap and available materials, and has moderate
reaction conditions, less three-wastes and better safety; industrialized production can be easily realized, the product purity is high, and quality is stable, thuscompletely meeting the using requirement of 2, 4, 5-trifluoro-phenylacetic-acid as a pharmaceutical intermediate.