The manufacturing process of cephalosporin nucleus (6R,7R)-7-amino-3-[[(l-methyl-1-H-tetrazol-5-yl)thio]methyl]-8-oxo-5-thia-1-azabicyclo[4,2,0]oct-2-ene-2-carboxylic acid hydrochloride (7-ATCA·HCl) was improved. Studies were made for the synthesis of cephalosporin nucleus 7-ATCA·HCl catalyzed by boron trifluoride complexes. Boron trifluoride acetonitrile complex or boron trifluoride Et ether complex, which were the traditional boron trifluoride complexes, were substituted by boron trifluoride di-Me carbonate complex to catalyze the reaction of 7-ACA and 5-mercapto-1-methyltetrazole to prepare 7-ATCH·HCl. Appropriate mole proportion of boron trifluoride di-Me carbonate complex and 7-ACA was determined, in which resulting product yield was exceeding 124% and assay was exceeding 99%. 7-ATCA·HCl prepared by this method which was obtained with high yield and good quality. In addition this method was suitable for com. production scales.