The invention discloses a downhole power cable insulation monitoring device comprising a low-frequency voltage source, first to fourth inductors, a Hall current sensor, a signal conditioning circuit, an A/D converter, a processing module, a display module, an alarm, and a communication module. The processing module comprises a processing unit and a determining unit. The signal conditioning circuit comprises a wave trap circuit, a filtering circuit, an amplification circuit, and a rectification circuit. The wave trap circuit comprises first to fourth resistors, a first capacitor, a second capacitor, and an operational amplifier. The downhole power cable insulation monitoring device may online monitor the insulation state of a cable in real time, improves coal mine power supply continuity, reliability and security, achieves high sensitivity and low errors, and timely notifies downhole and ground workers for carrying out handling so as to guarantee operation safety and prevent economic losses.