The invention belongs to the transformer on-line monitoring technological field and relates to a transformer core grounding current online monitoring device and method suitable for low-temperature operation, in particular, an online monitoring device and method used for transformer core grounding current and suitable for low-temperature operation. The device of the invention includes a housing, athermal insulation coating and a circuit board; the housing is located at the outermost layer of the monitoring device; the thermal insulation coating is applied onto the inner surface of the housing;the circuit board is located at the innermost layer of the device; the housing is divided into an upper shell and a lower shell; the upper shell is provided with a window and is sealed by tempered glass; a hole is formed in the top of the lower shell and is used for the wire leading of a core through type instrument transformer; the hole is sealed; the bottom end of the lower shell is provided with an aviation socket for connecting a power source line and a communication line; a mounting hole is formed in the lower shell; and a rubber sealing layer is arranged at the joint seam of the housing. The monitoring device of the invention can stably and reliably operate in a low-temperature environment. With the monitoring device adopted, workload can be reduced, maintenance costs can be reduced, and a faulty transformer to be handled in time, and accidents can be avoided. The monitoring device is of great economic and social significance.