Abstract: Addressing the operational and maintenance challenge of misjudgment in short-term ultrasonic partial discharge detection of 500kV GIS due to external transient interference, this study focuses on a suspected partial discharge hazard in the outgoing line bay of a 500kV substation. Ultrasonic and ultra-high frequency (UHF) live detection technologies are employed for signal acquisition and spectrum analysis. Through a comprehensive approach involving 40 minutes of long-term fixed-point monitoring, multi-instrument cross-validation, and full-domain point scanning retesting, it is determined that the initial anomaly was caused by external transient interference. A synchronous analysis mechanism combining "ultrasonic + UHF" is established, proposing a two-tier detection process of "short-term screening + long-term review" along with a multi-instrument cross-validation anti-interference method. Long-term prevention and control measures are formulated from four dimensions: detection standardization, equipment lifecycle management, on-site interference mitigation, and intelligent monitoring upgrades. These measures can significantly reduce the probability of misjudgment in partial discharge detection, providing a standardized practical scheme for similar GIS live detection and condition-based maintenance.