Abstract: Based on the abnormal test results of DC resistance and insulating oil chromatography for a 110kV transformer, it was judged that there might be discharge defects in the off-circuit tap changer on the medium-voltage side. Combined with the inspection with the cover removed, the fault points of contact surface oxidation and discharge on the off-circuit drum-type tap changer of the transformer were identified. The causes were analyzed, including insufficient operation of the tap changer, non-standard testing, and the influence of excessive harmonics. Moreover, the characteristics of off-circuit tap changers with different structures were compared, and suggestions for subsequent application were proposed.