Abstract: In the operation of the power system, various faults or abnormal operating conditions may occur at any time. The relay protection device can timely send out alarm signals when the electrical equipment is abnormal, or directly isolate and cut off the fault part. It is the most important defense line for power grid safety. The main transformer differential protection is the main protection means of the transformer. When a fault occurs outside the protection range of the main transformer, the main transformer differential protection should not act 1. In this paper, a main transformer differential protection action event caused by short-circuit trip of 10kV line in the near area is briefly analyzed, and the fault cause is confirmed, which provides reference for the investigation and analysis of similar problems in the future.