Provided is a method for preparing 2,5-dimethylphenylacetic acid, wherein p-xylene is mixed with paraformaldehyde and concentrated hydrochloric acid in a solvent of ion liquid to obtain 2,5-dimethyl benzyl chloride by the chloromethylation reaction. Then, 2,5-dimethyl benzyl chloride is introduced into a reactor with an acid binding agent and a solvent, the carbonylation and hydrolysis reaction is conducted in the presence of a catalyst to obtain 2,5-dimethylphenylacetic acid. The present process has new route, less synthesis steps, simple operation, lower cost, increased yield, and is friendly to the environment. Therefore, the method is suitable for industrial production.