Abstract: As a long-lived extreme toxic nuclide, ²³⁹Pu can enter the human body through inhalation, ingestion and wound infiltration, causing internal exposure to occupational personnel. Presently, urine sample measurement is generally used to evaluate internal exposure of occupational personnel. Existing methods for measuring plutonium in urine include low background alpha spectrometer, thermal ionization mass spectrometer (TIMS), inductively coupled plasma mass spectrometer (ICP-MS), and accelerator mass spectrometer (AMS). Because AMS method can achieve lower detection limit for the measurement of ²³⁹Pu, it is of great significance for accurate evaluation of the internal dose of occupational personnel. Using the self-developed AMS of China Institute of Atomic Energy (CIAE), the pretreatment process and the high-sensitivity measurement method of ²³⁹Pu in urine, whose detection limit is 1.1 μBq (0.47 fg), were carried out and it can provide technical support to ensure the radiation safety of occupational personnel.