Abstract: In order to study the productivity dynamic and soil water variations in the grasslands with alfalfas for different growth years, the productivity and soil water in the grasslands with alfalfas growing for three years, four years, six years, eight years, twelve years, eighteen years and twenty-six years were analyzed. The results show that there are very significant differences of yield with alfalfas for different growth years. There are regularity change of soil water contents in alfalfa field: soil water contents increase with the deeper soil depth, and tend to keep a stable soil water contents in soil deeper than 300 cm below soil surface. As the growth years of alfalfas are more than eighteen years, the soil water contents in upper soil begin to recover, but soil drying occurres through the deeper soil layer and soil moisture is still difficult to recover. Water use efficiency of alfalfa decreases with the increase of growth years of alfalfa. The research shows that in semi-arid areas of the Loess Plateau, the feasible growth years of alfalfa is eight years, and the period of the highest yield is the sixth year.