Abstract: Transformer is an important part of the power system, and the stable operation of the transformer is a necessary condition for the safe and stable operation of the power system. As one of the main protection of the transformer, the differential protection can protect most of the faults between the current transformer (TA) on each side of the transformer. But the action of the transformer differential protection is often not because of the failure of the primary equipment, and a large part of the situation is due to the secondary equipment caused by the differential protection action trip. So, this paper briefly explains the differential protection of the transformer, briefly describes the conditions of the differential protection trip, and then analyzes the differential protection error event caused by the secondary circuit problem.Put forward the improvement suggestions on the similar problems mentioned above.