The invention discloses a synthesis method of a parecoxib sodium impurity, namely N-[4-(5-methyl-3-phenyl-4-isoxazolyl)phenyl]sulfonic acid, belonging to the technical field of chemical pharmacy. The synthesis method comprises the step of carrying out sulfonation reaction and hydrolysis reaction by taking 5-methyl-3, 4-diphenylisoxazole as a raw material to obtain the parecoxib sodium impurity. The synthesized high-purity parecoxib sodium impurity can be used as a standard impurity in parecoxib sodium finished product detection and analysis, so that the accurate impurity location and qualification realized through the parecoxib sodium finished product detection and analysis are improved, the impurity can be better controlled, and furthermore the quality of a parecoxib sodium finished product is improved. The synthesis method disclosed by the invention is simple in operation; the raw material is cheap and available; and the yield of the obtained product is up to 95+/-5%, and the HPLC purity is larger than or equal to 98%.