Abstract: This study systematically elaborates on the practical application of oil chromatography diagnostic technology in transformer maintenance, clarifies the basic principles and diagnostic methods of this technology, and focuses on the specific applications of this technology in the maintenance of key components such as the iron core, winding, tap changer, bushing, oil tank cooling system, as well as the breather and oil level monitoring system. This includes fault identification and localization before maintenance, monitoring during the maintenance process, and effect verification after maintenance. By analyzing a maintenance case of a 220 kV transformer with multiple grounding faults in the iron core, this study verifies the effectiveness and accuracy of oil chromatography diagnostic technology in guiding transformer maintenance work, providing technical reference for condition assessment and maintenance decision-making of power equipment.