Abstract: Radioxenon isotopes are important monitoring components of comprehensive nuclear-test-ban treaty (CTBT). An anti-cosmic ray γ spectrometer was set up for measuring low-level active xenon isotope sample in CTBT verification regime. Compared with the original γ spectrum, the suppression factor of integrated background is about 6 times in the energy range of 20-2400 keV. Within 3 days measurement by anti-cosmic HPGe γ spectrometer, the minimum detectable activities of ¹³¹Xe^m,¹³³Xe^m,¹³³Xe and ¹³⁵Xe are 3.2, 4.3, 2.8 and 6.1 mBq respectively, which satisfies the requirements of CTBT.