Abstract: An LC-MS/MS method was established for the identification of the related substances in faropenem sodium.HPLC separation was carried out on Lichrospher ODS-3 column(250 mm×4.6 mm,5 μm) by gradient elution with a mobile phase consisting of 0.2% ammonium acetate and 0.5% formic acid aqueous solution and pure acetonitrile.The results were obtained by PDA and post-column acetonitrile aided positive ESI-MS and tandem mass spectrometry.Good resolution of faropenem sodium and its main related substances was achieved.Five related substances in faropenem sodium were detected by LC-MS/MS .Compared with the bulk pharmaceutical,the number and contents of the impurities in the preparations increased,especially after stress test in solutions with an elevated temperature.The established method is effective in the separation and identification of the related substances in faropenem sodium and the results are useful for its quality control.