Abstract: Abstract: In order to find out the best water-phosphorus (P) application method in improving grain yield and water use efficiency of winter wheat grown on the semi-arid Loess Plateau of China, this study evaluated the effects of phosphorus application at different soil depths on root length distribution, water use efficiency and grain yield of winter wheat under different water condition and analyzed the relationship between root distribution and grain yield. The experiment consisted of four P treatments and two water treatments in a randomized complete block design with three replicates. The four phosphorus levels included no P (CK), surface P (SP), deep P (DP) and deep band P (DBP) and the two water treatments were irrigation (W1) and no irrigation (W2) in the whole growth period of winter wheat. The results showed that there was large difference in root length distribution, water use efficiency and grain yield of winter wheat among different treatments. Compared with the irrigation treatment, the non-irrigation treatment significantly (P<0.05) lowered the surface soil root length (0-30 cm) by 17.5%, but significantly increased (P<0.05) deep soil root length (30-100 cm) by 13.3%, which would be helpful in promoting the absorption of soil water and P, water use efficiency and grain yield. Root length density was increased at the site of placement where P was applied under the two water treatments, particularly under water stress. Whether irrigation or not, P application (SP, DP and DBP) caused the increase of P uptake amount, water use efficiency and grain yield, compared with the CK treatment. The effect of P placement on water use efficiency and grain yield of winter wheat was different. Deep P application (DP and DBP) significantly (P<0.05) decreased water use efficiency of winter wheat compared with SP by 13.3%, while significantly (P<0.05) increased water use efficiency and grain yield under non-irrigation treatment by 28.5% and 16.0%, respectively. Furthermore, root length in deep soil layer (30-100 cm) was positively correlated with P uptake amount, grain yield and water use efficiency under non-irrigation condition，that is, the longer deep soil root length, the better water use efficiency and higher production under soil drought condition. No correlation was found between root length in deep soil layer (30-100 cm) and P uptake amount, grain yield and water use efficiency under irrigation condition, highlighting the importance of well-developed subsoil root system for both high grain yield and water use efficiency. The study suggested that deep placement of fertilizer phosphorus was a practical and feasible method of increasing grain yield and water use efficiency under water stress conditions by promoting deep root development.