Abstract: ⁸⁵Kr is a noble gas radioactive nuclide with a half-life of 10.75 years, and it decays by emission of beta rays associated with weak gamma rays. Most of the ⁸⁵Kr released into the atmosphere is artificially produced by neutron induced fission reactions, and it plays an important role in atmospheric environmental monitoring, detection of clandestine production of fissile materials, and verification of atmospheric transport model. Low-level detection of ⁸⁵Kr is therefore required. Internal gas proportional counter (IGPC) is one of the primary techniques to measure ⁸⁵Kr. However, studies about anti-coincidence measurement based on IGPC mostly focused on the measurement of ³⁷Ar, and few studies about ⁸⁵Kr were reported. In this paper, an anti-cosmic ray IGPC system was established to improve the detection sensitivity of ⁸⁵Kr. A quasi-4π well-type plastic scintillator surrounding the inner IGPC was used as an anti-coincidence detector to actively shield the background arising from cosmic rays. Compared with the non-anticoincidence spectrum, the background count rate of the anti-coincidence spectrum decreases from 3.64 s⁻¹ to 0.182 s⁻¹. The activity concentration of ⁸⁵Kr was measured using length compensation method, and the detection efficiency of the system was calibrated to be 95.8%. The minimum detectable activity of ⁸⁵Kr of about 7 mBq was achieved within 24 hours, which effectively improves the measurement sensitivity of ⁸⁵Kr.