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The application discloses a preparation method of 3-chloro-5-fluoro-2-methylbenzoic acid, relates to the technical field of 3-chloro-5-fluoro-2-methylbenzoic acid production, and the method is characterized in that: 2-methyl-5-nitrobenzoic acid is used as a starting raw material, selective chlorination is carried out on the starting raw material in preheated high-temperature concentrated sulfuric acid and dichloro hydantoin, then esterification is completed by a one-pot method, catalytic hydrogenation reduction is carried out under low-pressure and low-temperature conditions by the synergistic action of a Raney nickel catalyst and a platinum-carbon catalyst, a high-purity amino intermediate is obtained, smooth thermal decomposition fluorination is carried out in a diglycol dimethyl ether solvent, alkaline hydrolysis and refinement are carried out, and finally the end product is obtained; the product has high purity and high yield.