Abstract: Deficit irrigation changes the process of field water transformation. Few previous studies discussed the soil water dynamics below the crop root zone and its impact on crop water consumption. In this study, field experiments of winter wheat-summer corn rotation under deficit irrigation were conducted at the typical farmland in Beijing. The process of crop water requirements and water conversion under deficit irrigation were simulated by the SWAP model after calibration and validation. Furthermore, the optimal deficit irrigation modes under the condition of different hydrological years were obtained based on this model. The results indicated that deficit irrigation made crop consumed large amount of soil water. When precipitation and irrigation were small, the soil water consumption could be accounted for 46.1% of crop water consumption. There was obvious soil water exchange between root zone and storage zone with the range of soil water flux from -2.67 to 0.45mm/d. However, the water flux at the lower boundary of storage zone was small and changed a little. The percolation of the root zone always occurred after irrigation or larger precipitation, and soil water was supplemented from the storage zone upward into the root zone at the critical period of crop water requirement. Compared with conventional irrigation, the optimal irrigation modes could save irrigation water of 375mm, 225mm and 225mm, and the amount of drainage reduced up to 89%, 17% and 2% in the hydrologic years of 75%, 50% and 25%, respectively.