The water transportation and heat conduction in frozen soil is a complicated process due to the existence of phase transition.To describe the change law of water and heat in freezing process more precisely

a coupled heat-fluid transport mode for frozen soil was established based on the water flow in unsaturated soil.Both the finite element method and finite difference method were used in the discretization respectively

two different computer programs were written.Then the two programs are used to simulate a indoor frozen test.An error factor was proposed

and then the relationship between time step and error factor

total time of calculation was studied.Through the comparison of the real test and the two different calculation model simulation

it is shown that the finite element method possesses a better calculation stability.The modified adaptive time step method was proposed at last.The calculation result shows that

under the presupposition of keeping calculation accuracy of the model

the new method can reduce the total calculation time greatly.