The invention provides a novel method for preparing 2,4,5-trifluoro-phenylacetic acid. The novel method specifically comprises the following steps: (1) dissolving 1,2,4-trifluoro benzene into 1,2-dihalogenated ethane, carrying out a Friedel-crafts alkylation reaction under catalysis of a lewis acid catalyst, thus preparing 2,4,5-trifluoro halogenated ethylbenzene; (2) hydrolyzing the 2,4,5-trifluoro halogenated ethylbenzene obtained in the step (1) in an alkaline system, thus obtaining 2,4,5-trifluoro phenethyl alcohol; (3) oxidizing the 2,4,5-trifluoro phenethyl alcohol obtained in the step (2), thus obtaining the 2,4,5-trifluoro phenylacetic acid. The novel method disclosed by the invention has the advantages that raw materials are easy to obtain, the reaction is mild, the operation is easy, the cost is low, environment friendliness is realized, no high-toxicity reagent exists, and the like.