The utility model provides a monitoring method which accurately, rapidly and automatically judges occurrence of a looped network fault of two sets of direct current power supply systems in a transformer substation. A corresponding device includes a signal source, a I segment loop coupling circuit, a II segment loop coupling circuit, a current sampling circuit, a current signal processing circuit, an A/D conversion circuit, a single-chip microcomputer, a liquid crystal display, and an alarm circuit, so as to realize real-time monitoring and alarm of the two sets of direct current power supply systems when a looped network fault occurs, and if the looped network fault occurs, the alarm circuit acts to send a looped network fault alarm signal, thereby comprehensively satisfying requirement of an electric power transformation protection field.