In order to ensure the normal operation of the high-voltage power supply system, the high-voltage power battery pack needs to meet insulation safety requirements during normal operation. An improved method for on-line detection of insulation resistance and fault point location is proposed to solve the problems of low accuracy and poor reliability of insulation resistance detection of power battery packs, and inability to locate the internal insulation fault location of the battery pack. This method combines balanced bridge and unbalanced bridge to measure, improves the calculation method of insulation resistance, can quickly and accurately realize the insulation resistance detection of power battery pack, and can estimate the location of insulation fault inside the battery pack. According to the improved measurement principle, the hardware and software of the detection system are designed using microcontrollers, optocoupler relays, differential analog-to-digital converters, digital signal isolation, isolated power supplies, etc., to achieve insulation resistance detection and fault location. Experimental test show that the improved detection method and detection system can accurately detect the insulation resistance value, the detection error is within 5%, and the insulation fault point is located accurately, which provides a practical method for the insulation resistance detection and fault location of the power battery pack.