Abstract: The electrical characteristics of converter faults are quantified according to energy changing rate of fault signals, which characterize the variation of converter fault signal relative to that of normal state. According to the different variation of each electric quantity, the online monitoring electrical quantities that can localize and diagnose all kinds of converter faults effectively are determined. By comparing the energy changing rate of online monitoring electric quantities with zero, the logic code value of each online monitoring electric quantity is obtained in different converter faults. According to the different logic code values, online monitoring and diagnosis system of converter fault is presented by combing statistic knowledge. The simulation results show that the system can accurately localize and diagnose all kinds of converter faults, which are not influenced by system operation mode and transient resistance.