Desiccation of the deep soil layer under forest and grass vegetation is one of the crucial topics in scientific research for eco-environmental construction of the Loess Plateau.Soil water balance of grassland in gully regions on the Loess Tableland in various hydrological years was simulated numerically with simulated rainfall. The results show that soilwater balance of grassland with high vegetation cover ratio could be studied with WAVES model, and the simulated values of soil water storage capacity agree well with the measured values but how to determine correlative parameters need to be studied further. To keep in balance the supply and the demand of soil water, the rainfall amount during the rainy season and in the whole year should at least be 507.0 mm and 747.6 mm respectively, higher than the multi-year average value, respectively, indicating that soil water tends to have deficit in drought and average years on the Loess Plateau, leading to formation of the dry soil layer.