In view of the limitations such as close effective distance and low efficiency of the existing radioactive source detection methods, high-power microwave (HPM) is used to detect radioactive materials over a long distance because of its good spatial radiation characteristics. In this paper, the principle of microwave pulsed plasma breakdown and the influence of free electrons on breakdown characteristics are explained, the process of free electrons generated by the decay of radioactive source ¹³⁷Cs is analyzed, and the HPM breakdown time and breakdown threshold are presented. Based on the HPM atmospheric breakdown plasma experimental apparatus, HPM irradiation experiments were performed multiple times in the low-pressure environment of 6000 Pa, 7000 Pa and 8000 Pa with presence and absence of radioactive sources. Experimental results show that the presence of a radioactive source lowers the HPM breakdown threshold by approximately 10% and can reduce the breakdown time by approximately 50%.