Soil water evaporation is affected by many factors, but the main factors are soilwater and litter thickness. A mathematical model on soil water evaporation under littercover of 5 cm in thickness is: Q_t = (652.5e~(0.0166)t -660.5)/e~(0.0166)t. It has been foundthrough testings that this mathematical model can be used to describe the rule of soilwater evaporation, and its parameters are easy to be fixed and convenient to be used