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Water shortage is a serious problem in the arid oasis of Northwest China. Traditional flood irrigation wasted considerable amount of precious water, especially in Shiyang River Basin. Improving irrigation efficiency is vital for sustainable water resources management in the region. In this study, the DSSAT (Decision Support System for Agro-Technology Transfer) model was used to determine the effects of different irrigation scenarios on soil water evaporation, percolation, transpiration, grain yield and irrigation water use efficiency (IWUE) of spring wheat (Triticum aestivum L.) and maize (Zea mays L.). Prior to running the model, calibration was carried out using 4 years’ field experimental data and good agreement between simulated and measured crop phenology, grain yield and evapotranspiration (ET) were achieved. Long simulation results from 1985-2004 reveal the following: Considerable soil evaporation occurs under traditional irrigation in especially spring wheat (131.7-147.0 mm) and maize (79.8 mm) fallow period, accounting for 73.2-81.7% and 31.4% of annual precipitation, respectively. Under traditional irrigation, about 29.5-34.0% of irrigated water (675-745 mm) for wheat and 28.5% of irrigated water (765 mm) for maize percolates into deep soil layer from root zone, only less than 50% of the water is used by crop transpiration. Rational adjustments in irrigation schedule with 80 mm winter irrigation during fallow and 375 mm during wheat cropping season along with cancelling winter irrigation and supply with 440 mm irrigation during maize cropping season is recommended. In comparison with the traditional practice, 220-290 mm irrigation for wheat and 325 mm irrigation for maize can be saved without significant reduction in crop yield. Meanwhile, IWUE can be increased by 39.8-54.4% for wheat and 66.2% for maize in the study area.