Abstract: The mechanism of freezing damage of tunnels in the cold regions is complicated. These freezing damages are mostly related to water migration and phase transition in the soil. Based on the theory of mixture， a joint differential equation for the hydrothermal coupling problem of cold region tunnels considering water migration and water ice phase transition was established. The numerical simulation of temperature field and water field coupling was carried out by further development of COMSOL Multi-physics software， and the numerical simulation results were compared with the results of soil column freezing experiment to verify the effectiveness of the hydrothermal coupled numerical simulation model. Finally， taking the Milin Tunnel in Tibet Autonomous Region as an example， the temperature field and water field were simulated and analyzed， and the temperature field considering water migration was compared. The results show that with the increase of time， the temperature at the top boundary of the tunnel reduces from -0.82℃ to -9℃， the internal boundary temperature of the tunnel decreases from -0.74℃ to -11.11℃， and the tunnel temperature rises in March; the peak of ice content appears in January， and the ice content begins to decline in March. At the same time， the heat conduction velocity in the temperature field without considering moisture migration is faster， which proves that the influence of latent heat of phase transition on the distribution of temperature field in the tunnel is greater than that of liquid convection caused by gravity migration. The research results can reflect the occurrence process of freezing damage in the rich water tunnel in cold area， which has certain reference value.

Key words: tunnel in the cold regions, water ice phase change, moisture transfer, hydro thermal coupling, COMSOL software