This paper introduces the necessity of installing insulation monitoring device in high voltage DC(HVDC) system, analysis the shortcomings of the current insulation detection methods, proposed a perfect monitoring method, and emphatically introduces the principle and application of the method. This method can detect the bus voltage and the grounding resistance when bus or branch in the symmetrical grounding or not, so the reliability and the accuracy are improved. The results of multisim simulation and experiments show that the device can detect the insulation resistance accurately, and then find out the fault branch, it is very convenient for fault analysis staff. Finally, some suggestions are put forward for the use of the device.