Abstract: Radioactive xenon isotope is one of important components for International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT). It is a difficult problem that the activity of radioactive xenon isotope should be accurately measured. The principle of β-γ coincidence efficiency extrapolation method was described in detail in this paper. And the activity of ¹³³Xe was absolutely determined by β-γ coincidence efficiency extrapolation method. In conclusion, β-γ coincidence efficiency extrapolation method can be applied to measure the activity of gas radionuclide. For the sake of increasing accuracy, β detector efficiency should be enhanced by decreasing gas self-absorption effect.