NOVELTY - Preparing 2-nitro-4-(methylsulfonyl) benzoic acid, comprises (i) adding 2-nitro-4-methylsulfonyltoluene as raw material into a reactor at 5-75 atmospheric (atm) pressure at temperature of 110-450 degrees C, introducing air, oxygen enriched air or oxygen as an oxidizing gas, and adding less amount of unsaturated carboxylic acid as a solvent, and adding heteropolyacids -transition metal-halides forming multi-homogeneous catalytic systems; (ii) performing gas-liquid reaction process; (iii) performing detection reaction, and crystallizing; and (iv) performing catalytic oxidation reaction.    USE - The method is useful for preparing 2-nitro-4-(methylsulfonyl) benzoic acid (claimed).    ADVANTAGE - The method can overcome the production of waste gas and waste water pollution during the preparation process, and thus it environmental friendly; and prevents severe corrosion, low selectivity and high production cost of the product.    DETAILED DESCRIPTION - Preparing 2-nitro-4-(methylsulfonyl) benzoic acid, comprises (i) adding 2-nitro-4-methylsulfonyltoluene as raw material into a reactor at 5-75 atmospheric (atm) pressure at temperature of 110-450 degrees C, introducing air, oxygen enriched air or oxygen as an oxidizing gas, and adding less amount of unsaturated carboxylic acid as a solvent, and adding heteropolyacids-transition metal-halides forming multi-homogeneous catalytic systems; (ii) performing gas-liquid reaction process of reaction solution containing 2-nitro-4-methylsulfonyltoluene, and oxidizing the reaction solution to 2-nitro-4-methylsulfonylbenzoic acid; (iii) performing detection reaction, and filtering 2-nitro-4-methylsulfonylbenzoic acid product, crystallizing, and separating the precipitate from the reaction solution, and concentrating the filtrate by continuously recycling; and (iv) adding 2-nitro-4-methylsulfonyltoluene and unsaturated carboxylic acid, and performing catalytic oxidation reaction, where the heteropolyacids are phosphotungstic acid, silicon acid, phosphomolybdic acid or molybdovanadophosphoric acid, and the transition metal manganese, iron, cobalt, zirconium, nickel, zinc, copper, titanium or chromium metal element, and halides are fluorine, chlorine, bromine, iodine, halides of acetylenetetrahalide, dihaloethane, halothane, ethane, hydrogen halide or potassium halide, and the transition metal may be derived from a metal salt dissolved in a solvent, oxide or hydroxide, and the metal salt is acetate, propionate, butyrate, nitrate, chloride salt, carbonate or sulfate.