The invention belongs to the technical field of anti-inflammatory drug production, and relates to a method for preparing mefenamic acid. The method comprises: adding o-chlorobenzoic acid and 2,3-dimethylaniline into a system which is formed by a non-protonic polar solvent and a dehydrant, performing condensation reaction in the presence of an acid-binding agent, a catalyst and a phase-transfer catalyst, and obtaining mefenamic sodium; acidifying the mefenamic sodium and obtaining coarse mefenamic acid products; and refining the coarse mefenamic acid products in an organic solvent and water and obtaining finished mefenamic acid products. The method improves the production efficiency of the mefenamic acid and reduces the production cost of the mefenamic acid.