Abstract: OBJECTIVE To establish a high performance liquid chromatography-tandem mass spectrometry(LC-MS/MS) method for determining astragaloside IV in rat plasma and to compare pharmacokinetics of Astragalus injection in normal and hyperthyroid model rats. METHODS The plasma sample was handled by protein precipitation. The column was Ultimate^TM XB-C₁₈ column(4.6 mm×50 mm, 5 µm) with the mobile phase of water and acetonitrile by gradient elution at 0.6 mL·min^-1. The column temperature was 30℃. The electrospray ion source with positive ion mode and select the ion reaction monitoring mode were used. RESULTS The linear range of Astragaloside IV was 5-1 000 ng·mL^-1. The precision, accuracy, mean recoveries, matrix effects and stability were satisfied with the requirements of biological sample measurement. Compared with the pharmacokinetic parameters of normal group, the AUC and MRT of astragaloside IV decreased significantly with the CL_z and V_z increased significantly in hyperthyroid model rats. Compared with normal rats, the AUC decreased and the rate of metabolism was significant acceleration in hyperthyroid model rats. CONCLUSION The established LC-MS/MS method for the determination of astragaloside IV in rat plasma is simple, sensitive, accurate and feasible, which is suitable for the pharmacokinetic study of Astragalus injection in rats.