Abstract: The mechanism of fertilization in increasing water use efficiency(WUE) has been studied by the field experiments with nitrogen, phosphorus and farm manure in the east area of subhumid lands prone to drought, Gansu province, from 1991 to 1995. The experiment results indicate: The winter wheat root growth can be stimulated by fertilization, especially by combination of nitrogen and phosphorus, so it may be extended the space of water uptake by roots. When nitrogen and phosphorus couple, they may supplement one another in the growth stages and the function. Compared with no fertilization, on an average, the evapotranspiration of the treatments with fertilization increased by 5.4%, but the yields and WUE increased by 92 8% and 79 7%,respectively. Fertilization may stimulate the growth of canopy of winter wheat, increase the amount of transpiration and decrease the amount of evaporation, so it may enhance the ratio of transpiration to evaporation, WUE and productivity of water.