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. The reliable operation of the transformer differential protection is related to whether the transformer has the ability to remove the transformer faults quickly and reliably, so as to ensure the stable operation of the power system. Therefore, it is necessary to analyze the reliability of transformer differential protection. In addition, as a substation operator, it is also very important to handle the accident after the main transformer failure. The ability to report the accident quickly and reliably and quickly fault isolation is crucial to the stability of the power system. The following analyzes the differential protection principle, differential protection method and accident treatment process.KEY WORD: Principle of differential protection；secondary loop；differential protection and preventive measures.