By water balance method and a series of large lysimeters we measured and studied the ground water utilization amount of rainfed crop and its rule of change at different ground water levels which are controlled autdmatically. Along wilh the rise of ground water level, the used ground water amount increased progressively according to a minus power exponential curve approximately, when the depth of ground water is between 0.9 and 1.5 meter, the ground water utilization amount of rainfed crop accounts for 30%-50% of the evapotranspiration of the winter wheat and transplanted cotton during the same period. In shallolw ground water areas, the ground water used by rainfed crop plays an important role in irrigation agriculture.