The emphasis of this study is laid on effect of alfalfa growth on soil water eco environment based on data obtained from actual measurements in a long term experiment.At first, yield of fresh alfalfa and its change with the growth life, and the relationship between grass yield and precipitation were analysed.Then, the signif icance of measuring soil water at depth for calculating consumption and revealing effects of alfalfa on water eco environment were presented.Difference in calculating soil depth resultsed in different conclusions in analysis of effects on water eco environment.It pointed out that in the Loess Plateau the annual consumption is more than the annual precipitation for alfalfa which has deep roots more than 10 meters.Therefore cont inual growth of alfalfa can lead to desiccation and form a desiccative layer below the upper soil layer, which affects the way of water cycle on land.It is concluded that in order to slow down or prevent formation of the drying layer an appropriate productivity goal of grassland should be set and relevant agricultural technological measures are proposed.