Abstract: When replacing the DC system insulation monitoring device in a power plant, it was found that the measured voltage continued to change when using a multimeter to measure the DC system positive and negative busbar-to-ground voltages after dismantling the original insulation monitoring device. This paper briefly describes the working principle of the insulation monitoring device, and analyzes the principle of measuring the positive and negative busbar-to-ground voltages with a multimeter when the insulation monitoring device is installed and not installed. Advise when the DC system insulation monitoring device is withdrawn.