This paper studied the dynamics of alfalfa growth and its relation with the excessive depletion and resumption of soil water in loess hilly and gully region. The results showed that alfalfa could grow in this region for 10 years, being most flourishing at the forth and fifth year, but declined gradually because of the excessive depletion of soil water. From 0 to 35 cm above ground, the stem biomass was higher than the leaf biomass, and the ratio of stem to leaf was 1.7:1; at 40 cm above ground, the biomass of stem and leaf was equal; and from 45 to 90 cm above ground, the leaf biomass was higher than the stem biomass, with the ratio of leaf to stem 1.42:1. The soil dry layer was not distinct in first two years of alfalfa growth, but getting thicker with time, being from 110 cm in third year to 260 cm in seventh year, with a 4.6%-6.2% of soil water content. It took 5 years for the natural resumption of soil water in degraded alfalfa grassland, and soil water content was increased with time.