Groundwater resources have been used with growing intensity since the 1980s in the Northern Plain of China due to lack of water. As a result, the farm land groundwater level has decreased and the storage capacity of soil has increased accordingly in the well irrigation areas. It is necessary to take measures to reduce the loss of surface flow and evaporation and to use flood and runoff water to recharge the local groundwater. Four year data on rainfall, groundwater and soil moisture from the water resources experimental area in the south eastern suburb of Beijing were processed in order to study the dynamic characteristics of soil water under the condition of a deep groundwater table. This data was divided into three phases for each year—the period of spring irrigation, the rainy season and the period of the end of autumn to the beginning of next spring, in order to analyze the dynamic characteristics of soil moisture in different seasons. The analysis showed that during the dry season the watering or precipitation (<80 mm) was primarily stored in 0~1 m soil layer. During the rainy season, when the top soil is relatively damp, if a rainstorm (100~150 mm) occurs the water will be primarily stored in 1~2.5 m soil layer without surface flow appearing. The research showed that the soil reservoir had very good regulation capacity and 85% of the normal annual precipitation was stored in the 3 m soil (named soil reservoir). Therefore if appropriate measures are taken on farmland before a heavy rainstorm occurs, then there will be minimal runoff appearing and most of the precipitation will be stored in the soils.