A rapid, sensitive and selective liquid chromatography-tandem mass spectrometry (HPLC-MS-MS) method has been developed for the rapid separation and structural identification of constituents in Astragali Radix (AR) The analysis was conducted on an ACQUITY HPLC chromatographic instrument and a mass spectrometer using positive electrospray ionization. Using a fast HPLC system with an ACQUITY HPLC BEH C18 column, the total analysis time for this complex herb was less than 30 min. The method used a column with 1.7 µm particle packing, which allowed a higher speed of analysis, peak capacity, greater resolution and increased sensitivity. With various fragmentor voltage levels in MS, accurate mass measurements (less than 5 ppm error) for molecular ions and characteristic fragment ions represented reliable identification criteria for these compounds. The constituents of AR were identified or tentatively characterized based on their retention times, mass fragmentation behavior, MS-MS fragment ions, literature reports and the establishment of an in-house molecular formula database. With this method, a total of 22 compounds of AR were tentatively identified based on MS data and comparison with available databases. In conclusion, fast HPLC with MS is a highly useful and efficient technique to separate and identify constituents in complex matrices of herbal medicines.