Abstract: Substation transformer refers to the step-down transformer for substation and hydropower station, which is an important equipment in power system. Most substation transformers use insulating oil as insulating medium, so the test data of insulating oil can reflect the performance of insulating oil, thus further reflecting the internal operation of oil-immersed substation transformers. If the test data is abnormal, there will be faults in the substation transformer, which will provide targeted reference for subsequent maintenance. Therefore, this paper introduces the reason analysis of an abnormal test data of 35kV substation variable body insulating oil.