Abstract: In the report, based on the active low-voltage pulse signal injection method, the mathematical calculation model of insulation resistance was re-derived, and which was used to design an active power battery insulation resistance monitoring system. The system can solve effectively the problems that some methods cannot accurately measure simultaneous decrease of the insulation resistance of the positive and negative terminals and positive and negative unilateral insulation resistance. Additionally, the effectiveness of the monitoring system was verified by bench experiments, which has certain application values in improving electric vehicle power battery monitoring system.