Abstract：According to the in-situ measured data of the air temperatures and ground temperatures inside at Fenghuoshan Tunnel on Qinghai—Tibet railway，the thawed range of the permafrost surrounding the tunnel is extensive，which is caused by the hydration heat of the cast-in-place concrete lining，insulation measurement in the inlet，exit of the tunnel during winter construction，and other human activities. Taking water transfer and phase change into account，the finite element formulae of this problem and computation program are derived from the governing differential equations and water transfer equations by Galerkin's method. With the computer program，forecast and analysis on the refreezing of permafrost for Fenghuohan Tunnel is performed. The analytical results show that the vault，left wall，and right wall of Fenghuoshan Tunnel will be refrozen in three years，and the arch bottom will be refrozen in September 30 of the fifth years without insulation. On the other hand，if the tunnel is fitted with insulation with thermal conductivity of 0.03 W/m·K，the vault of Fenghuoshan Tunnel will be refrozen in three years，left wall，right wall and the arch bottom will be refrozen at September 30 of the fifth years. Therefore，insulation material will delay the refrozen of permafrost of Fenghuoshan Tunnel. The thawed range of the permafrost surrounding the tunnel in cold regions caused by construction should be considered，and the time of observation should prolong when their temperatures and stresses are observed in site，otherwise observations will differ largely from the real values in the stable state of the tunnel.
