In this paper, based on the measured data in field, the regularity of evapotranspiration with droughts and soil water deficit conditions has been analysed. Based on soil water balance equation, and comprehensively consideving over the weather condition. Soil moisture state and crop's bioloyical property, we established the mathematics model for the computation of eva-potranspiration from the farmlunds when soil moisture supply was restricted. This model in-volves three parameters, that is, the initial soil water content θ₀, the evaporation potential E₀ and crop leaf area index LAI, and at the same time, the sensitivity of actual evapotranspira-tion from the farmlands to evaporation potential, the initial soil moisture content and leaf area index were also analysed. Also in this paper, two coefficients in; widely-used penmen formula at present were mo-dified by using the data measured at the radiation stations in Shanxi, Shaanxi, Gansu and Nei Mongol, etc. Finally, the error with the method presented in this paper base been analysed, and the reliability with the model has been evaluated. The result of comparing the estimated evapotr-anspiration with measured evapotranspiration at the experiment plots in Shaanxi and Gansu provinces is that this method has good precisim, it can be used in the arid and semiarid areas in China.