Abstract: The generator circuit breaker (GCB) serves as a core protection device for large-scale hydroelectric generating units. Leakage of its SF?insulating gas will directly degrade the equipment’s insulation and arc-extinguishing performance, even trigger locking of the opening function, and pose a severe threat to the safe and stable operation of the power station. Taking the SF?leakage defect in the arc-extinguishing chamber of the HEC-7S GCB for Unit #3 in a large-scale hydropower station as the research object, this paper systematically sorts out the whole process of the defect from occurrence, on-load emergency treatment to standardized maintenance during shutdown. Based on equipment disassembly results and operational historical data, it conducts an in-depth analysis of the root causes of the defect and reveals the aging and cracking mechanism of the support insulator under the coupled effect of long-term mechanical load and switching vibration. Combined with engineering practical experience, a long-term prevention and control system for SF?leakage defects of GCB equipment in large-scale hydropower stations is constructed from four dimensions: condition monitoring, emergency treatment, maintenance management and control, and full life cycle management. The research results can provide technical references and engineering demonstrations for defect treatment, daily operation and maintenance, and risk prevention and control of GCB equipment in similar hydropower stations.