The preparation and isolation for impurities (2R,6R,7S)-3-((carbamoyloxy)methyl)-7-methoxy-8-oxo-7- (2-(thiophen-2-yl)acetamido)-5-thia-1-azabicyclo[4.2.0]oct-3-ene-2-carboxylic acid (A)，(6R,7S)-3- ((carbamoyloxy)methyl)-7-methoxy-7-((R)-2-methoxy-2-(thiophen-2-yl)acetamido)-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid (B)，(6R,7S)-3-((carbamoyloxy)methyl)-7-methoxy-7-((S)- 2-methoxy-2- (thiophen-2-yl)acetamido)-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid (C) of cefoxtin were firstly described. These impurities are characterized by NMR and HRMS spectral data. These findings could be important for quality control and validation of the analytical method in the manufacture of cefoxitin.