Abstract: In equipment applications with complex service conditions, stringent requirements are imposed on high temperature resistance and moisture resistance of winding wires. To study insulation performance of glass-fiber wrapped winding wires (hereinafter referred to as glass-fiber wrapped wires) in high-temperature and humid environments, their performance degradation under environments with different relative humidity levels was investigated. Characterization methods such as scanning electron microscopy (SEM) and contact angle measurement were employed to analyze the causes of the decrease in insulation performance, with a focus on exploring the evolution process of the insulation layer interface. Insulation resistance tests showed that when relative humidity increased from 55% to 75% at 55 ℃, the resistance of all samples exhibited a rapid decay trend, and the decline rate of thermally aged samples (8%-18%) was significantly higher than that of unaged samples (3%-5%). Breakdown voltage tests revealed that the breakdown voltage of unaged samples was consistently higher than that of thermally aged samples, showing an overall decrease with increasing relative humidity, while thermally aged samples experienced a maximum decrease of up to 30.27%. Contact angle analysis indicated that increased relative humidity caused the surface contact angle to decrease from 122.5° to 114.5°, enhancing hydrophilicity, and thermally aged samples were more severely affected by this change. SEM analysis results demonstrated that in high-relative humidity environments, only microcracks measuring 2 μm appeared in unaged samples, whereas the cracks in thermally aged samples increased to 15-35 μm, accompanied by delamination and resin detachment.