Disclosed in the utility model is an on-line circuit fault monitoring and protecting device that comprises a phase-splitting breaker. The phase-splitting breaker is connected with an isolation switch that is connected with a transformer station bus; and the transformer station bus is connected with a PT cabinet. In addition, the phase-splitting breaker is respectively connected with a low-impedance reactor and a grounding protection control unit; the low-impedance reactor is connected with a current transformer that is connected with a first grounding grid; the grounding protection control unit is respectively connected with the low-impedance reactor, the current transformer and the PT cabinet; and the transformer station bus is connected with a second grounding grid. According to the on-line circuit fault monitoring and protecting device provided by the utility model, on the basis of the effective connection of the phase-splitting breaker, the low-impedance reactor, and the current transformer, a fault point can be found out rapidly, thereby ensuring the personal safety of the maintenance personnel, substantially improving the fault eliminating efficiency, and ensuring the normal power supply for residents.