Abstract: Carbon, hydrogen and oxygen isotope tracer techniques were used to analyze theseasonal dynamics of water sources and water use efficiency of poplar (Populus alba) in Zhenglanqi, with direct judgment method and multivariate mixed linear model. The results showed that there was obviously seasonal dynamics in poplar’s water sources. In early and late stages of growing season (April-May, October), poplar used both shallow soil water (0-50 cm) and deep soil water or underground water under 150 cm. In middle stage of growing season (June to September), poplar primarily used precipitation which infiltrated to 0 to 150 cm soil. Water use efficiency of poplar was relatively higher, with 35.90, 13.90, 20.65, 28.93 and 63.88 mmol C·mol^-1 H₂O from May to September, respectively. Poplar could use deep soil water and increase water use efficiency when shallow soil water could not satisfy its water requirement in rainy season. When there was enough precipitation, poplar use soil water from the top layer, and water use efficiency decreased gradually as well. These results indicated that poplar had higher water use efficiency due to its ability to use alternative water sources and controlling its water use rate in semiarid area. These strategies assist poplar to reach the greatest water use efficiency favoring their growth.