The invention discloses a novel method of preparing 2-nitro-4-methylsulfonyl benzoic acid. The target product which is 2-nitro-4-methylsulfonyl benzoic acid is prepared by carrying out reaction by adopting an oxidation reaction device with a self-priming stirrer, wherein 2-nitro-4-methylsulfonyltoluene is used as the raw material, sulfuric acid is used as a reaction medium, a transition metal oxide is used as the catalyst, and oxygen is introduced in the course of reaction. The method provided by the invention has the advantage that the reaction condition is easy to control, the production cost is low, and the amount of the generated three wastes is reduced.