The invention discloses an on-line monitoring device of three mutual inductors of a grounding current of a clamp of a transformer. The on-line monitoring device of the three mutual inductors of the grounding current of the clamp of the transformer solves the problems that testing data deviation on the grounding current of the clamp of the transformer by the prior art is large and safe operation of the transformer is influenced. The on-line monitoring device of the three mutual inductors of the grounding current of the clamp of the transformer comprises a clamp grounding outgoing line supporting insulator (13) fixedly arranged on the outer side of the tank body of a transformer oil tank (1), one end of a clamp grounding outgoing line (8) is connected with a clamp grounding connecting sheet (4) through an outgoing line sleeve (7), the other end of the clamp grounding outgoing line (8) is connected with a grounding end (6), the clamp grounding outgoing line (8) is arranged in a coil of a first current transformer (10), the two ends of the secondary coils of the three mutual inductors are connected to the two input interfaces of an AD conversion interface circuit (14) respectively, and the output end of the AD conversion interface circuit (14) is connected with a single-chip microcomputer (16). The relative accurate grounding current monitoring value of the on-line monitoring device of the three mutual inductors of the grounding current of the clamp of the transformer provides basises for operation and maintenance personnel to take effective protection measures.