Evaporation effect on the suction in the unsaturated soils is analyzed by solving the equations of isothermal flow and coupled heat-moisture anisothermal flow with various calculated models of evaporation. The influences of various climatic parameters and soil properties on the calculation of evaporation are also discussed. It is shown that the behavior of suction in the unsaturated soil can be perfectly modeled by the equation of coupled heat-moisture anisothermal flow with the formulation of considering actual evaporation and commonly-used methods by solving isothermal flow with the boundary of potential evaporation evidently overestimate the evaporation magnitude in the soil surface and the decreasing magnitude of suction. The major governing factor for the evaporation of soil surface is outer climatic condition where solar net radiation and wind velocity are the most contribution

while the internal parameters of soil property play a less role in the estimation of evaporation.