At present, the mechanical characteristics of lining structure of high geothermal tunnel are not clear. The field test and thermalmechanical coupling numerical simulation are carried out to study the mechanical properties and the safety characteristics of secondary lining under high geothermal environment of Sangzhuling Tunnel on SichuanTibet Railway. Meanwhile, the load design for high geothermal tunnel is proposed. The results show that: (1) The stress of secondary lining varies rapidly in 10 days and becomes stable after 20 days. (2) The maximum tensile stress locates at crown, while the maximum compressive stress appears at sidewall. (3) The load correction coefficient of high geothermal tunnel can be expressed as the polynomial relationship of the initial temperature of surrounding rock. (3) The stress of secondary lining shows an increasing trend with the increase of the surrounding rock temperature, but there is a difference in the rate of increase at each point; the tensile stress fluctuates with the temperature. (4) The minimum safety factor appears at crown and decreases with increasing temperature of the surrounding rock; when the temperature is higher than 60 °C, there is a possibility of damage. (5) The maximum crack width locates at the crown, and it increases as the temperature increase.