Abstract: This paper introduces the overheating defect of a 35 kV transformer caused by the loosening of the soft connecting bolt at the bottom of the bushing at the low-voltage side. Through the analysis of the three ratio method of chromatogram of the fault transformer oil, it is preliminarily determined that the fault is of high temperature and overheating type. The winding overheating defect is further judged by the auxiliary analysis of"four ratio method". Then conduct power failure high-voltage test on windings at high and low voltage sides of the transformer. Finally, it was found that the DC/T ratio at the low voltage side exceeded the standard. Through disassembly inspection, it was found that the soft connecting bolts at the bottom of the low voltage bushing were loose. The preventive measures are put forward.