Abstract: In the research of water-crop, it is important to estimate the evapotranspiration exactly. Based on SPAC (Soil—Plant—Atmosphere Continuum) theory, one academic model to estimate evapotranspiration is introduced. By this model, evaporation, transpiration and evapotranspiration can be calculated by the data of weather, such as air temperature, vapour, radiation, wind speed and the sunlight, and crop(such as Leave Area Index, height and leave width) and soil surface (moisture and temperature) factors. In this model, the vapour of the vegetable stoma is assumed as saturated vapour and the vapour of the soil surface is gained by the experiential method. Through the soil water transfer model, the method can be validated by comparing the calculation and the examination of the soil water. The analyses indicate that the evapotranspiration have close relation with the soil and the crop, except for the weather, and the Leaf Area Index influence the proportion of the soil evaporation. Moreover, it can be proved that this model accords with the Penman Model and the Shuttleworth-Wallace Model in substance. Since all the input data can be gained in the field experimentation, this model can be applied widely.