Abstract:  Fenghuo Mountain tunnel,lying in permafrost,is a key project in Qinghai-Tebit Railway.The temperature distributions of this special tunnel will directly affect the stability of tunnel structure.Based on the governing differential equations of the problem of temperature field with phase change,the finite element formulae of three-dimensional temperature fields are obtained from the Galerkin’s method.Three-dimensional temperature fields of Fenghuo Mountain tunnel in coming 50 years are forecast on the basis of the air temperature measured inside the tunnel.A comparison between the numerical results and the measured temperature of the rock surrounding the tunnel is made.The results show that the theory,model,parameters and conditions are all credible,and that there will not occur a seasonally thawed range in the rock surrounding Fenghuo Mountain tunnel in coming 50 years.It can provide theory basis and a numerical method for actual engineering design in cold regions.