Provided is an on-line monitoring and fault diagnosis system for a power distribution network PT. The system can be used for measuring three phase currents, three phase voltages, three line voltages and zero sequence voltage data of the power distribution network PT. Whether the system has a single-phase grounding fault, a ferromagnetic resonance fault and a PT line breakage fault can be accurately judged through actually-measured data. The system is composed of a current measurement terminal, a collector and a background server. The current measurement terminal is arranged on a three-phase line of the power distribution network PT, and the three phase currents of the power distribution network PT are measured according to the electromagnetic induction principle. The collector is arranged on a panel of a PT cabinet, receives data of the terminal, meanwhile, measures the phase voltages, the line voltages and a zero sequence voltage of the PT, and uploads the data to the background server. The background server is arranged inside a transformer substation, receives the data of the collector, and conducts on-line monitoring and fault diagnosis on the power distribution network PT. According to the on-line monitoring and fault diagnosis system for the power distribution network PT, technology is mature, and reliability is high.