Tazobactam is an important β-lactamase inhibitor developed by Japan Taiho pharmaceutical company. The impurity of six-membered ring isomer in the cyclization reaction of tazobactam synthesis was isolated by column chromatog. and analyzed by 1 H-NMR, 13 C-NMR, 1 H-1 HCOSY, HSQC, HMBC and DEPT135.