Abstract: The valid irrigation water amount, calculated by traditional method of field water use efficiency (FWUE) based on the theory of sufficient irrigation, can not reflect the water transfer and utilization in field as it includes invalid water. In this paper, the validity of evapotranspiration was analyzed according to the crop-water production function, and a new calculation method of FWUE was built based on the theory of deficit irrigation. Calculated by the new method, the FWUE of rice was 0.768 in flooding-irrigation mode, and 0.915 in Wet Irrigation mode. The result indicate a more accurate picture of water transfer and utilization in field.