Abstract: In high-altitude areas, high-intensity ultraviolet (UV) radiation can induce changes in the molecular structure of silicone rubber, thereby leading to significant aging of composite insulators, ultimately resulting in insulation performance failure. However, existing studies rarely address the physicochemical characteristics of composite insulators in strong UV environments. Moreover, meteorological parameters exhibit stochastic variability in natural environments in contrast to the fixed parameters in laboratory settings. This leads to significant discrepancies between the UV radiation aging patterns observed in controlled experiments and under natural conditions. Accordingly, this study focused on the performance evolution and intrinsic mechanism of composite insulators in UV radiation environments, selecting Qinghai, a high-altitude region with strong ultraviolet radiation, and Shandong, a region with relatively weak ultraviolet radiation intensity, as research areas. The insulators that have been in operation on the grid for different number of years in both regions were the research objects in conducting a comparative analysis of the effects of UV aging on the physicochemical properties and electrical performance of insulator materials. Through molecular dynamics simulations, the fracture mechanism of Si—O—Si bonds under the UV-electrothermal coupling effect was further explored. The results indicate that in high UV regions, the contents of Si—(CH₃)₂, C—H in (CH₃) and Si—CH₃ groups all exhibit decreasing trends with increasing operational duration. However, for Si—O—Si groups, the dominant role between oxidative crosslinking and cleavage processes during different operational stages differs, whereby their content exhibits first an increasing and then decreasing variation trend with the extension of service life. In contrast to Qinghai Province, all group contents in Shandong Province decreased with operational duration. Concurrently, with increasing operational duration, the O/Si atomic ratio of composite insulators in Qinghai Province increased from 1.19 to 1.37, whereas the contents of Si(–O)₃ and Si(–O)₄ groups increased from 23.48% and 11.50% to 58.84% and 21.75%, respectively. Under the same operational duration, the insulators in Qinghai Province exhibited higher O/Si ratios and greater Si(—O)₃ and Si(–O)₄ contents than those in Shandong Province. These results indicate that intense UV radiation accelerates methyl group loss and surface inorganic transformation processes. Consequently, the static contact angle of the insulators in Qinghai Province decreased by 22.1%, compared to only 6.3% in Shandong Province. With increasing operational duration, silicone rubber crosslinking structures of composite insulators in both regions were damaged, accompanied by surface morphological deterioration. This damage introduced physical and chemical traps on the surface, leading to a decrease in flashover voltage. In Shandong Province, the crosslinking structures of the insulator shed exhibited more severe damage, resulting in a higher density of physical traps. The increased accumulation of the charges on the insulator surface caused a greater disruption to electric field uniformity compared to the insulators in Qinghai Province. Consequently, under the same operational duration, the insulators in Shandong Province demonstrated lower flashover voltages than those in Qinghai Province. Under the combined effects of electric field and temperature, alterations occurred in Si—O—Si bond energy. When the bond energy drops below 427 kJ·mol^–1, UV radiation induced Si—O—Si bond cleavage. The findings of this study provide theoretical support for the aging assessment and operational maintenance of composite insulators in strong UV regions.