【正文】 ature of the surrounding rock .In order to cope with this problem，we simultaneously solve the two groups of equations based on the fact that at the tunnel wall surface both temperatures are equal .We should bear in mind the phase change while solving those equations concerning the temperature of the surrounding rock，and the convection while solving those equations concerning the temperature of the air flow, and we only need to smooth those relative parameters at the tunnel wall surface .The solving methods for the equations with the phase change are the same as in reference [3]. Determination of thermal parameters and initial and boundary conditions Determination of the thermal parameters. Using p= H，we calculate air pressure p at elevation H and calculate the air density using formula , where T is the yearlyaverage absolute air temperature，and G is the humidity constant of air. Letting be the thermal capacity with fixed pressure, the thermal conductivity，and the dynamic viscosity of air flow, we calculate the thermal conductivity and kinematic viscosity using the formulas and. The thermal parameters of the surrounding rock are determined from the tunnel site. Determination of the initial and boundary conditions .Choose the observed monthly average wind speed at the entry and exit as boundary conditions of wind speed，and choose the relative effective pressure p=0 at the exit ( that is，the entry of the dominant wind trend) and on the section of entry ( that is，the exit of the dominant wind trend )，where k is the coefficient of resistance along the tunnel wall, d = 2R，and v is the axial average speed. We approximate T varying by the sine law according to the data observed at the scene and provide a suitable boundary value based on the position of the permafrost base and the geothermal gradient of the thaw rock materials beneath the permafrost base.3 A simulated example Using the model and the solving method mentioned above，we simulate the varying law of the air temperature in the tunnel along with the temperature at the entry and exit of the Xiluoqi Tunnel .We observe that the simulated results are close to the data observed[6]. The Xiluoqi No .2 Tunnel is located on the Nongling railway in northeastern China and passes through the part beneath the permafrost base .It has a length of 1 160 m running from the northwest to the southeast, with the entry of the tunnel in the northwest，and the elevation is about 700 m. The dominant wind direction in the tunnel is from northwest to southeast, with a maximum monthlyaverage speed of 3 m/s and a minimum monthlyaverage speed of 1 .7 m/s . Based on the data observed，we approximate the varying sine law of air temperature at the entry and exit with yearly averages of 5℃，℃ and amplitudes of ℃ and ℃ respectively. The equivalent diameter is 5 .8m，and the resistant coefficient along the tunnel wall is the effect of the thermal parameter of the surrounding rock on the air flow is much smaller than that of wind speed，pressure and temperature at the entry and exit，we refer to the data observed in the Dabanshan Tunnel for the thermal parameters. Figure 1 shows the simulated yearlyaverage air temperature inside and at the entry and exit of the tunnel pared with the data observed .We observe that the difference is less than 0 .2 `C from the entry to exit.Figure 2 shows a parison of the simulated and observed monthlyaverage air temperature inside (distance greater than 100 m from the entry and exit) the tunnel. We observe that the principal law is almost the same，and the main reason for the difference is the errors that came from approximating the varying sine law at the entry and exit。2,observed values parison of simulated and observed air temperature inside The Xiluoqi Tunnel in ,simulated values。 that is we assume that T=f(x)3%on R=Ro. We find that, after one year, the heat flow trend will have changed in the range of radius between 5 and 25m in the surrounding rock.. Considering that the rock will be cooler hereafter and it will be affected yet by geothermal heat, we approximately assume that the boundary R=Ro is the second type of boundary。2,outside air temperature year when permafrost maximum thawed depth after Begins to form in different permafrost formed in different years Sections of the surrounding rock4 .3 Preliminary conclusion Based on the initialboundary conditions and thermal parameters mentioned above, we obtain the