Abstract: To explore the root cause of the reboot and shutdown phenomenon of electronic equipment in a strong electromagnetic field environment, this study considered a certain type of DC-regulated power supply as a test object and observed the susceptibility phenomena exhibited by the power supply under strong continuous wave electromagnetic radiation. In this experiment, the relative variation in voltage was selected as the effect parameter, and the variation feature of the effect parameter with the interference field strength was described. The irradiation test was carried out in the GTEM cell with an interference signal frequency range of 80–1000 MHz and a maximum field strength of 300 V/m. The test results indicate that the output voltage variation of the test power supply can be divided into two stages. When the interference field strength was low, the variation types of voltage included monotone rise, monotone fall, or first rise and then fall. At this stage, the voltage variation did not exceed 20%, and the load equipment could still operate normally. When the interference field strength was high, the voltage variation showed a sudden change. It could be divided into three types of interference phenomena such as shutting down after stopping interference (80–120 MHz, 320–350 MHz), shutting down when jamming (220–270 MHz, 360–420 MHz), rebooting (570–590 MHz, 700–720 MHz, 860–880 MHz), which posed actual threat to the load electrical equipment. There was no obvious correspondence between the susceptibility phenomena of the two stages.