Abstract: Discussion on the technical scheme for testing the insulation performance of transformers in substation commissioning. For typical defects such as poor grounding of the casing end screen and deterioration of insulation oil, insulation resistance segmented measurement and multi frequency tan δ scanning are adopted as the core detection methods. The polarization index（PI=R600s/R60s） obtained through the ZC-90 tester was used to accurately locate the abnormal contact resistance of the end screen. Combined with the QS30 dielectric loss meter, it was detected in the frequency range of 50～200 Hz that the medium polarity impurities in the oil caused a dispersion rate of 20%. The handling measures include replacing the spring contact finger grounding structure and implementing a dual stage vacuum+membrane filtration combination process to reduce the contact resistance to 45 mΩ and the micro water content to 8ppm. The leakage current step up test showed an abnormal increase in current at 0.8U_n. By injecting BHT antioxidant and distributed fiber optic temperature measurement system, the aging process of the medium was effectively controlled.