The invention relates to an ungrounded system single-phase earth fault monitoring device and method, and belongs to the field of power system fault monitoring. The primary side of a single-phase voltage mutual inductor in the device is connected with any one and only one of a phase A, a phase B and a phase C through a primary side fuse, and the other end of the primary side is grounded; one end of a secondary side of the single-phase voltage mutual inductor is connected with one end of a parallel suppression resistor through a secondary side fuse, and the other end of the secondary side is connected with the other end of the parallel suppression resistor and grounded; an overvoltage relay is connected with the two ends of the parallel suppression resistor; a low-voltage relay is connected with the two ends of the parallel suppression resistor. The method includes the steps that firstly, the ungrounded system single-phase earth fault monitoring device is arranged, then a setting value of the overvoltage relay and a setting value of the low-voltage relay are set, and finally fault judgment is carried out. The device is simple in structure and low in cost. Voltage to earth of any phase is used for judgment, and judgment is not influenced by the magnitude of voltage and current and is not influenced by the unbalanced current of a zero-sequence current filter.