This article addresses the challenge of detecting internal structural defects in ultra-high voltage GIS isolation switches. An innovative high penetration X-ray detection technology is applied, which is based on a 7.5 MeV electron cyclotron and can achieve visual diagnosis of internal equipment abnormalities. In practical engineering applications, this method successfully discovered the defect of partial detachment of the PTFE sealing strip at the overlapping shielding of the isolation switch’s moving contact seat. To evaluate the potential harm of the defect, the study further combined electric field simulation to analyze the electric field distribution of the strip under different detachment states. The simulation results show that in the partially detached state, the electric field distortion coefficient is low, the maximum field strength is much lower than the critical breakdown field strength, and the discharge risk is controllable. This technology route that combines advanced non-destructive testing technology with numerical simulation not only accurately locates hidden defects, but also scientifically evaluates their safety, providing important technical support and decision-making basis for on-site operation and maintenance of ultra-high voltage GIS equipment.