Prunus armeniaca is a main shrub species of ecological and economic plantation in semi-arid area in western Liaoning Province. A water use strategy, including the main water source which a plant species utilizes and water use efficiency (WUE), plays an important role in plant survival and growth. Understanding the water use strategies of a shrub plantation can be used to evaluate the sustainability and long-term stability of the plantation. Stable oxygen isotope ratios (δ18 O) of water in rain, well, branch xylem, and in soil at different depth of 5, 11- and 17-year-old P. armeniaca plantation were tested to trace the main water source of P. armeniaca. Root systems were investigated to determine the depth of the main distribution. Leaf carbon isotope ratios (δ13 C) were used to evaluate the long-term WUE of P. armeniaca plantation. The results showed that 5- and 11-year-old P. armeniaca plantations mainly derived soil water from the 0—30 cm layer, which were coincident with the distribution of their fine roots. Conductive root systems of 5- and 11-year-old P. armeniaca plantations were distributed primarily in the 20—50 cm layer. 17-year-old P. armeniaca plantations derived water mainly from the 0—20 cm and 50—70 cm layers, with their fine root and conductive root system being distributed primarily in the 0–30 cm and 10–70 cm layer, respectively. The δ18 O values indicated that these sources of soil water were recharged by rainfall and groundwater. The long-term WUE of 5-year-old plantation (-24.1‰) was greater than those of the 11- and 17-year-old plantations (P＜0.05). The water use strategies suggested that P. armeniaca plantation can adapt to a semi-arid environment of western Liaoning Province by changing its water use strategy over time.