Abstract: Based on the theory of energy balance of soil surface,micro-meteorology,moisture and heat transfer in unsaturated soil,an evaporation model is established.Then a fully implicit finite difference method with automatically adjusted time step is developed for simulation.In this model,Neumann condition and Cauchy condition are introduced for the upper boundary of moisture and heat transfer,respectively.These made the model both stable and efficient.The model has been tested using the data of Yerqiang River Basin in North-West China for 31 days.Then numerical simulations are performed to examine the effects of soil type, depth to ground water table and weather conditions etc.It shows that the model can be applied to simulate the soil moisture and heat transfer in long period,and can also be used to find out the relationship between evapor ation and external environments.