Abstract: Vegetation carrying capacity of soil water is the key of the ecological environmental construction and sustainable development in the Loess Plateau. Based on local monitoring data of rainfall, runoff, soil moisture dynamics and Biomass under condition of 4 different sites in loess region of Northern Shaanxi from 2008 to 2010, relationships between natural precipitation and soil water supply, soil water supply and biomass, biomass and soil water consumption were analysed and established in this paper, and Soil Water Carrying Capacity of alfalfa had been separately calculated by FAO method and water balance method. The results showed that: in alfalfa land replenishment of soil moisture in alfalfa land and biomass showed a linear relationship; biomass and soil moisture consumption was a quadratic function. By using FAO method, it could be estimated that the maximum alfalfa yield of soil moisture can bear was 3 992.2 to 4 173.7 kg/hm2 in Northern Shaanxi's Loess Plateau; however, the biomass was estimated by water balance method was 2 600 to 3 500 kg/hm2, it was 16.07% to 33.52% less than the result estimated with FAO method. With the same aspect and position, capacity reduced while slope was increasing. With the same aspect and position, capacity downhill was larger than that uphill. With the same slope and position, capacity in southern slope was smaller than that in northern slope.