Abstract: Radionuclide verification is one of the four monitoring technologies used in International Monitoring System of Comprehensive Nuclear-Test-Ban Treaty (CTBT). The activity of radionuclides concerned by CTBT generally performs at the low-level. An anti-cosmic ray γ spectrometer system with a HPGe detector was developed by using a combination of active and passive shielding techniques in order to improve the detection sensitivity and reduce the affections of background. By adjusting the system with a delay time and testing anti-coincidence count loss correction, compared with the original γ spectrum, the integral background in the energy range of 15-2 700 keV is reduced by a factor of 8. The detection sensitivity is greatly improved by using the anti-coincidence method. For ¹⁴⁰Ba, the minimum detectable activity (MDA) is 10 mBq by using the anti-cosmic ray γ spectrometer for 7 d measurement, which meets the technical requirements for certification of nuclide laboratory under the CTBT. It is not only of great significance for CTBT radionuclide verification, but also important for radiation environment monitoring.