Abstract: To address the degradation of electrical insulation performance of third rail insulator brackets used in high-altitude mountain rail transit systems under extreme conditions such as low air pressure, pollution, and icing, this study employed a multifunctional artificial climate chamber to simulate the complex operating environment along the Sichuan-Tibet railway and systematically carried out flashover characteristic tests. The results show that the flashover voltage decreases monotonically with decreasing air pressure. When the pressure drops from 95.4kPa to 56.9kPa, the pollution flashover voltage of clean brackets decreases by 33.6%, as low-pressure conditions intensify arc drift and promote arc development. When the salt density increases from 0.5mg/cm2 to 3mg/cm2, the flashover voltage decreases by 43.56%, due to accelerated dryband formation and arc extension. The increased salt density significantly enhances the conductivity of the pollution layer's liquid film, thereby lowering the flashover voltage. Additionally, the flashover voltage gradually decreases with increasing ice thickness, primarily because ice ridges bridge and shorten the creepage distance of the insulator bracket. Furthermore, ice ridges distort the electric field distribution, with the most severe distortion occurring at the ridge tips. To further investigate this phenomenon, breakdown voltage tests were conducted under low air pressure using both needle-plate and plate-plate electrodes. The results reveal that the breakdown voltage under non-uniform electric fields is significantly lower than that under uniform fields, and insulation degradation is more pronounced in low-pressure environments.