To investigate the flavonoids in Platycladi Cacumen.The constituents in Platycladi Cacumen were determined by UPLC-MS. A Waters BEH C18 column (2.1 mm x 150 mm, 1.7 microm) was used with a gradient elution of methanol-water containing 0.2% formic acid. The mass spectrometer equipped with electrospay ionization source was used as defector and operated in data was collected under the negative ion modes.Eleven constituents were identified.In this study, the main flavonoids in Platycladi Cacumen were separated by UPLC, and identified through the information of mass number and UV spectra. With the information of MS/MS from flavonoids, the modes of breaking up flavonoids were discussed. It is an accurate and effective method which can be applied for the constituent identification of Platycladi Cacumen.