Abstract: Water is the most important factor affecting plant growth, primary production and ecosystem stability in rocky mountain areas in north China. In the rocky mountain areas in north China, there are three main plant species including Platycladus orlentalis, Quercus variabilis and Vitex negundo var. heterophylla, and they adapt to the dead soils well in such arid region and play an important role in vegetation regeneration in Beijing mountain areas. In order to understand the water sources of these species after rainfall in rainy season, the direct inference of hydrogen and oxygen isotopes and multisource mass balance assessment were used to estimate the water sources of these plant species based on the characteristics of stable isotopes of rainwater, soil water, groundwater and plant xylem water. The results were as follows: ① After rainfall, Platycladus orlentalis used mainly the upper soil (0-10 cm) water (35.8%) and the deepsoil (deeper than 80 cm) water (55.6%); ② The water sources of Q. variabilis were similar to those of P. orlentalis, the utilization rate of deepsoil water was higher than that of P. orlentalis,but that of uppersoil water was lower. The utilization rates of shallowsoil water and deepsoil water were 17.7% and 77.2% respectively; ③ The water sources of  Q. variabilis were the most complicated, but the proportion of uppersoil (0-10 cm) water was still dominant (46.6%). Our findings revealed that there was a difference of water use modes among plant species, thus the effective utilization of soil water was improved, the interspecific water competition in the semiarid ecosystem was reduced, and these were important for the species selection in ecological regeneration in the rocky mountain areas in north China.