Low temperature and anoxia are the main problems of highway tunnel construction in western Sichuan plateau. Because there is no design code for high-altitude tunnel, the design of thermal insulation and oxygen supply grade of tunnel lacks basis and exists blindness. The oxygen partial pressure of trachea, the mean temperature of the coldest month and the maximum freezing depth were selected as the three grading indexes. Based on the long-term monitoring of different altitudes of three typical tunnels and six tunnel sites in the western Sichuan plateau and the comparative analysis of the existing meteorological data, the corresponding relations between the three grading indexes and the altitude were clarified. The classification indexes of altitude are established as 2 100 m and 4 200 m respectively, and the highway tunnels in the western Sichuan plateau are divided into general altitude, high altitude and ultra-high altitude. The corresponding measures of dewatering, freezing prevention and oxygen generation and supply at different elevations are given. The research results can provide a basis for the design and construction of new tunnel in western Sichuan plateau, reduce the construction period of the tunnel, and ensure the service life and operation safety of the tunnel.