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This paper compared air gap and tolerance voltages correc-tion methods on high altitude area based on Sichuan-Tibet railway. With test results of short air gap in different areas of China, we verified the adaptability of insulation gap cor-rection method at an altitude of 4000 to 4500m. As a re-sult, we formulated a correction method and value of air external insulation under 50Hz overvoltage and lightning impulse overvoltage of 27.5kV traction power supply sys-tem at an altitude of 4000m to 4500m.