This paper used the theory of systems engineering and comlined the theory with practice to evaluate the main factors (include the land, sunlight, heat, and natural precipitation) of the natural ecological system of the dry farming on the Loess Plateau in terms of the potential productivity. And thereupon a scientific basis for exploiting the natural productivity potential of the dry farming in the region was proposed. This paper also analysed the controllable and uncontrollable factors of the ecological and economical system of theory farming and discuss-sed various forms of the reproductive behaviors of human economy. According to these studies, it made a scientific explanation for the great potential in the natural productivity of the dry land and for a common saying that "the poorer a man becomes the more land he reclaims ;and the more land a man reclaims, the peorer he becomes," which is concluded from historical facts. At last, it put forward a way of thinking that may be practical and worth choice in exploiting the agricultural resources of the dry land.