The invention provides a method for preparing diphenyl ether tetraformic acid and belongs to the technical field of organic synthesis. A polar aprotic organic solvent is used as a solvent, and the method is characterized in that: N-methyl-4-nitrobenzene o-dicarboximide serving as a main raw material participates in a pressure reaction in the presence of a catalyst and a compound synergistic catalyst for 5 to 10 hours when heated to 140 to 220 DEG C; pressure is relieved; the hot catalyst is filtered out; the solvent in the mother liquor is separated by distillation; water is added; cooling and filtering are performed to obtain an intermediate; the intermediate is hydrolyzed under the strong alkali condition; the hydrolysate is crystallized under the acidic condition; filtration, washing and drying are performed ; and a product is obtained.