Based on weather, leaf transpiration rate, leaf area index, effective rooting depth, soil moisture, soil evaporation and other data from the observed datum of Otingag sandy area, authors applied dual crop coefficient method to simulate daily crop water requirements of Elymus in 2004 and 2005. The consistency of crop water requirements was tested by comparing the simulated crop transpiration and evaporation with the measured values. On this basis, authors discussed the relationship between water requirements and yields, between photosynthesis and transpiration, between photosynthesis and root layer soil moisture content. The results showed that: the most direct way to saving-water should be soil moisture of effective root layer maintained at field water-holding rate of 51% to 74% for Elymus.