Abstract: As a core device in UHV flexible DC transmission systems, the operational stability of converter transformers directly determines the safety of the entire system. Oil leakage is a common fault during operation; if not addressed promptly, it may lead to low oil level, reduced insulation performance, or even major accidents. Based on the oil leakage case of the high-end Y/Y-C phase converter transformer at a ±800kV converter station, this paper systematically analyzes the root causes through on-site inspection and operational data review. A targeted treatment process of "oil drainage-disconnection-jacking repair-reset verification" is proposed, along with optimized operation and maintenance suggestions. The results show that the failure of the fixed limit round steel, causing uneven pressure on the tank edge sealing strip, is the fundamental reason. After applying the proposed process, the sealing performance fully meets standard requirements, with no leakage during one month of continuous operation. This study provides a reliable technical reference for diagnosing and handling oil leakage defects in UHV converter transformers.