Power Electronic Transformers (PET) face increasing risks of insulation discharge and breakdown under the combined effects of high-frequency, high-voltage, fast rise-time pulse electrical stress, and elevated temperatures. In order to study the impact of high-temperature degradation on insulation under real operating conditions, typical insulation polyimide (PI) films of transformers were exposed to temperatures ranging from 25 ℃ to 190 ℃. Subsequently, a simulation of PET operating conditions was conducted through the application of 20 kHz, 4 kV, 300 ns pulse voltage to assess insulation partial discharge and breakdown voltage. Time-resolved partial discharge spectroscopy (TRPD) and discharge statistical parameters were utilized to characterize the extent of insulation degradation. The results indicate an increased dispersion of discharges with rising temperatures; the number of discharges, average discharge amplitude, and maximum discharge amplitude increased by 233.3%, 277.3%, and 109.1%, respectively, with temperature elevation. The breakdown voltage distribution of the PI film conforms to the Weibull distribution, with the breakdown voltage decreasing from 4060 V at 25 ℃ to 3587 V at 190 ℃, representing an 11.7% decrease, with a gradually slowing decline rate. Therefore, the temperature influence on PET insulation performance was elucidated, providing crucial guidance for PET insulation fault diagnosis and reliability analysis.