Abstract: Water-adjustment of black soil in the farmland has a function to adjust crop water-demand during their growing stage. Water-adjustment capacity of black soil in the farmland is determined both by rainfall and water demand of crops during their growing stage. Time of water-adjustment by the soil mainly occurs in early stage of crop growing.Amount of water-adjustment by the soil accounts for 70% of crop water-consumption.A depth of water-adjustment by the soil is from 0 to 100cm,accounting for 80% of the total.The peak water-consumption stage of spring wheat and soybean is in the middle and late growth periods, and water-consumption mainly depends on rainfall. The peak water-consumption stage of corn is in the middle growth periods, and water-consumption mainly depends on rainfall too.