Abstract: The article introduces an inter-turn short-circuit defect in a 35 kV transformer winding caused by the high-voltage side winding deformation. After the differential protection of the transformer tripped, the staff conducted Dissolved Gas-in-oil Analysis (DGA) and concluded that the equipment had defects. Then, a series of power outage diagnostic tests were conducted to further qualitatively analyze, and conclusions were drawn that there may be winding deformation and interturn short circuits. In order to verify this conclusion, the transformer was returned to the factory for disassembly inspection, and it was finally found that the A-phase winding was deformed and there was inter-turn discharge. Finally, maintenance suggestions and opinions were proposed.