The invention discloses a method for synthesizing 2,4-dichlorphenoxyacetic acid. The method comprises the following steps of: reacting o-chlorophenol which is used as a raw material with chloroacetic acid to generate o-chlorophenoxyacetic acid; and then reacting o-chlorophenoxyacetic acid with chlorine in a mixed solvent to obtain 2,4-dichlorphenoxyacetic acid, wherein the mixed solvent is a mixture of DMF (dimethyl formamide), water and dichlorethane. According to the method, o-chlorophenol is used as the raw material, so that o-chlorophenol can be used, and a new train of thought is provided for the synthesis of 2,4-D; and the raw material is easily available and low in cost, so that the production cost of a product is low. By adopting the mixed solvent, the selectivity of reaction is improved, so that a catalyst is not needed; and through the matching of other conditions, the reaction temperature and other conditions ensure that the yield of the product is improved, the generation of 2,6-dichlorphenoxyacetic acid and 2,4,6-trichlorophenoxyacetic acid is greatly reduced, and the yield of 2,4-dichlorphenoxyacetic acid in a single step can reach over 95%.