The field simulation column method was employed to study quantitatively the dynamic law of soil moisture availability to plant in the southern Loess Plateau, by using a methematical modle on the relation between crop yield and soil moisture and a mathematical modle of soil moisture absorption by plant roots. The same results were achieved by the above-mentioned mathematical modles, i. e., when the soil moisture changes from 95% to 55% of the field capacity, with the soil moisture decresing gradually, the soil moisture availability to plant decreased parabolically, dropping faster and faster. Meanwhile, the reasons resulting in different viewpoints on the soil moisture availability and the condition in which the soil moisture availability is in agreement with the soil muisture easy-absorbability are discussed briefly.