Abstract: An emergency tripping of a 110 kV transformer was analyzed. Through the disassembly inspection of the faulty GIS equipment, it was confirmed that there were tiny defects (bubbles or impurities) in the insulation transmission pull rod, which caused the insulation capacity of the insulation pull rod to decrease during the long-term operation, which made the overall electric field of the disconnector uneven, and then caused internal discharge to cause AB phase-to-phase short circuit. Splash debris and vaporized metal conductors polluted the basin insulator, and finally caused three-phase short circuit. The external performance was transformer differential protection action, and the No.2 transformer tripped. In view of this incident, it is proposed to strengthen the live detection of GIS equipment, implement the "dust-free" production and assembly work site, and strengthen the supervision and management of key nodes by equipment owners and professional departments, which can find hidden defects early and eliminate them as soon as possible, and ensure the safe and stable operation of important equipment in substation.