Abstract: The diagnosis and analysis of a ±660kV converter transformer fault is introduced. The dissolved gas in the oil of the converter transformer was found abnormal through the oil chromatogram on-line device, and the nature of the transformer fault was determined by adjusting the load. In the blackout check process, according to the structural characteristics of the converter transformer, a temperature rise test was carried out according to different connective positions of columns; the oil chromatogram analysis was conducted by oil samples. The location and causes of the fault were initially determined. The validity and accuracy of the test was verified by disassembly inspection, which can provide reference for diagnosis and locating of similar transformer faults.