Abstract:The section from Ya’an to Kangding of the Sichuan-Tibet railway passes through the Xianshuihe fault. The fault zone develops not only geological structure, but also hydrothermal activity strongly, which makes the construction process of the tunnel crossing the fault zone face complex and changeable problems of hydrothermal disasters. To accurately predict the heat damage, it is necessary to deeply analyze the characteristics of the hydrothermal cycle and migration channel. However, due to the complexity of the hydrothermal system, the research results of thermal reservoir characteristics and hot water migration mode in the fault zone cannot be applied to the prediction of heat damage in tunnel engineering. Based on the comprehensive analyses of water-rock balance, silicon-enthalpy characteristic, chlorine-enthalpy characteristic, and other enthalpy and mass relations of the hydrothermal system, the multistage thermal reservoir characteristics of the hydrothermal system were deduced, the correlation between the thermal reservoirs was analyzed, the thermal reservoir temperature was calculated, and the conceptual model of the migration and cooling process of the hot water from Yulingong to Daofu section was established. The cationic temperature scale and silicon-enthalpy relationship was used to calculate the direct thermal reservoir temperatures of Yulingong, Erdaoqiao, and Zhonggu hot water, which were close in the range of 228～242 ℃, while the temperature of Bamei and Daofu was obviously low. The chlorine-enthalpy relationship showed that the upwelling separation position of Erdaoqiao and Zhonggu hot water was shallower than that of Bamei and Daofu hot water.?The mixing degree of the Erdaoqiao hot water system was the highest. The analyses showed that the hydrothermal system from Yulingong to Daofu takes Yulingong as the center, and the hot water moved and cooled along the passage at different depths in the northwest direction through the compound process of adiabatic cooling, heat conduction and mixing, forming multiple thermal hazard areas of tunnel engineering with different types and degrees of thermal damage. When the tunnel passes through the Xianshuihe fault zone from different parts, the characteristics of hydrothermal activity involved were different, and the types of thermal damage encountered and the degrees of danger were different. It is reasonable to choose area Ⅲ between Erdaoqiao and Zhonggu as the location where the railway crosses the Xianshuihe fault zone, but it is necessary to stay clear of the water-bearing rock stratum mixed with cold and hot water and pay attention to the drainage of plenty of hot water during engineering construction.