The invention relates to an online monitoring system capable of monitoring transformer core grounding current at extremely low temperature. The online monitoring system comprises an acquisition unit, a ground copper bar lead wire, a current transmitter, a processor, a display unit and an alarm system, and is characterized in that the acquisition unit is a cold-proof sensor composed of a sensor shell, a heating unit, an electronic device, a sealant, a lead wire and a heating power supply; the sensor shell is prepared from metal; the electronic device is placed in the middle of the inside of the sensor and comprises the current transmitter; the heating unit composed of one or more layers of heating nets which are composed of thermal resistors is arranged on the periphery of the electronic device; and for powering the heating unit, the lead wire is led out of the sensor shell and is connected with a corresponding heating system's heating power supply. The gap between the heating unit and the electronic device and that gap between the heating unit and the sensor shell are both filled with the sealant. The online monitoring system provided by the invention can safely and reliably operate at extremely low temperature, and operating ambient temperature of the system can reach minus 55 DEG C.