Research purposes: Ya,an -Kangdingsection of theproposed Sichuan - Tibet Railway would pass through South - East branch of the Xianshuihe fault zone，where thermal harm of hydrothermal mobile is prominent，which could influence on the selection of the railway plan. Currently， the calculation methods are unsatisfactory for geothermal gradient of hot water and temperature of the tunnel bottom plane. In order to make progress， this paper tried to use 11 inversion method about geothermal gradient of hot water" and 11 fitting method about temperature contour of the tunnel bottom plane11 for solution and propose corresponding geology route selection suggestion，which presented the new views on it for providing the reference. Research conclusions:( 1 ) The geotliermal gradient of hot water at the Dapingslian No. 2 tunnel and the Maojiagou tunnel is calculated and the temperature contour of the tunnel botom plane is drawn by 11 inversion method about meothermal gradient of hot water" and "fitting method about temperature contour of the tunnel bottom plane" methods， thermal harm influence zone is divided and field drilling results verify the rationality of t!ie contour map，which shows that these two methods is feasible. (2 ) Based on division of the thermal harm influence zone of the hydrothermal mobile，the author estimated thermal harm influence，then proposed corresponding geology route selection suggestion in hydrotliermal mobile zone. (3) The two methods of 11 inversion method about geothermal gradient of hot water" and "fitting method about temperature contour of the tunnel boom plane" can be used for reference for similar tunnel engineering sites which are in the hydrothermal mobile zone.