Abstract: To improve the standard of quality control of tazobactam and its preparations in China, national reference standard of tazobactam impurity A was developed. After tazobactam impurity A was synthesized, its structure was validated by infrared (IR), mass spectrometry (MS) and nuclear magnetic resonance (NMR), and its content uniformity and short-term stability were measured and investigated. Then, water content and residue on ignition of impurity A were determined, and its purity was determined using high performance liquid chromatography (HPLC) with 10 mmol/L ammonium acetate solution-acetonitrile (98∶2) as the mobile phase. Mass balance method was used to determine the content of the first batch of tazobactam impurity A national standard substance. Meanwhile, nuclear magnetic quantitative method was used to calculate the content, which was mutually verified with the mass balance method. The developed reference material of tazobactam impurity A is consistent with the maximum degradation impurity in tazobactam system applicability solution and the reference material of tazobactam related substance A contained in USP41. Within the 95% confidence range, the ratio of inter- and intra-bottle variance of impurity A after separation was 0.61 (< F_0.05(11,12)), proving that the uniformity was satisfying. The contents of organic impurity, water content and inorganic impurity in impurity A were 0.90%, 1.24% and 0.25%, respectively. The content of impurity A was determined to be 97.6% by mass balance method, which was basically consistent with the result of nuclear magnetic quantitative method (97.1%). Under the condition of 25 °C, the area normalized purity of impurity A was 99.1% at 0, 3, 5 and 10 days, proving that the sample was stable at room temperature for 10 days. Finally the first batch of national standard substance of tazobactam impurity A was established successfully.