In the quality study of cefminox sodium sterile, a new impurity was identified by HPLC-MS. Its structure was speculated to be sodium (6R,7S)-7-[[[[(2S)-2-amino-2-carboxyethyl]thio]acetyl]amino]-3-[[(1-methyl-1H-tetrazol-5-yl)thio]methyl]-7-methylthio-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate, and it was synthesized via substitution, acylation, deprotection and substitution from benzhydryl (6R,7S)-7-amino-7-methoxy-3-[(1-methyl-1H-tetrazol-5-ylthio)methyl]-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate and methanthiol. The structure was confirmed by 1H NMR and MS. It had the same peak retention time as the new impurity of cefminox sodium.