Abstract: Radioactive gas xenon is one of the important components for International Monitoring System (IMS) of Comprehensive Nuclear-Test-Ban Treaty (CTBT). It is a difficult problem that the activity of radioactive xenon can be accurately measured. The principle of β-γ coincidence efficiency extrapolation method was described in detail in this paper. The activity of ¹³¹Xe^m was absolutely determined by β-γ coincidence efficiency extrapolation method, and its standard uncertainty of measurement is 0.3%. According to β-γ coincidence efficiency extrapolation method, parameter method formula of ¹³¹Xe^m activity was deduced. And ¹³¹Xe^m activity result difference of these two methods is 2.0%. In addition, the activity of ¹³¹Xe^m was determined by HPGe γ spectrometry, and the discrepancy of measurement results between coincidence efficiency extrapolation method and HPGe γ spectrometry is 2.7%. The above results indicate that the measurement accuracy of ¹³¹Xe^m activity is improved significantly by β-γ coincidence efficiency extrapolation method.