After comparing the pros and cons of various isolation monitoring methods, this article presents the isolation monitoring method which based on unbalanced bridge and the characteristics of high-voltage battery system. Battery is an important component for Hybird-electric vehicle. The safety of high-voltage battery system is important specification for Hybird-electric vehicle. This device which uses Freescale’s 16-bit microcontroller with CAN communications capabilities achieve real-time and accurate measurement of isolation resistance of high-voltage battery system. The article focuses on the working principle of the device, hardware design and software design. This device has high accuracy, real-time, reliability and good working environment adaptability. The measuring accuracy meets the requirement of actual project.