On 22 June 2020, the trace amount of ¹³⁴Cs, ¹³⁷Cs and ¹⁰³Ru in the atmosphere were detected by RN63 radionuclide station (in Sweden) of the International Monitoring System (IMS) for the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Finland and Estonia declared the observation of these radionuclides and ¹⁴¹Ce, ⁹⁵Zr and ⁹⁵Nb respectively from 14 to 22 June 2020. Regarding the source term of the detection, various views all over the world were expressed on the networks. In this paper, these radionuclides and their activity concentrations are analyzed, and the nature of the detective event is determined through the analysis of these monitored data and the historical monitored data of each station. In addition, the release time of these radionuclide is estimated, and possible source term regions are obtained by using an atmospheric transport model. The results show that the activity ratio of ¹³⁴Cs to ¹³⁷Cs is about 1.10, which can exclude the possibility that the radionuclides detected come from nuclear test or nuclear explosion. The detection of ¹³¹I and ¹³³I at the IMS RN61 station may be related to the small defection of reactor fuel assembly, and correlated temporally with the detection of ¹³³Xe at the IMS RN49 station. Preliminary conclusions indicate that the ¹³⁴Cs, ¹³⁷Cs and ¹⁰³Ru are likely to be related to the release of improper handling of reactor maintenance or fresh spent fuel transfer in Northern Europe region. Furthermore, due to the activity concentration is only μBq/m ³order of these radionuclides, it has no effect on public health.