The utility model discloses an on-line monitoring system capable of monitoring the grounding current of a transformer core in an extremely low temperature environment, and the system comprises a collection unit, a ground copper bar lead, a processor, a display unit, and an alarm system. The interior of the collection unit is provided with a current transmitter. The system is characterized in that the collection unit is a cold-proof sensor which consists of a sensor housing, a heating element, an electronic device, sealing glue, a lead, and a heat-supply power source; the sensor housing is made of metal; the electronic device is placed at the center of the interior of the sensor, and comprises the current transmitter; the periphery of the electronic device is provided with one or more heating units consists of heating meshes which respectively consists of a thermal resistor; the power supply of the heating unit is achieved through that the lead is used for leading the heating unit out of the sensor housing to be connected with the heat-supply power source of a heating system; and sealing glue is filled between the heating unit and the electronic device, and between the heating element and the sensor housing. The monitoring system provided by the utility model can operate safely and reliably in the extremely low temperature environment, and the operating temperature of the system can reach 55 degrees below zero.