Abstract: In order to understand water source of the dominant tree species Quercus variabilis and its ecophysiological adaptation during growing season in Beijing mountainous area, we studied the water use strategy of Q. variabilis under different soil moisture conditions during growing season. We employed the Iso-source software and measured the D and 18O in branches, soil and groundwater to clarify their water source during growing season, by combining with ecophysiological factors, such as leaf water potential (), stomatal conductance (Gs) and water use efficiency (WUE). The results showed that the Q. variabilis had different water sources with season varying: from April to June, it mainly utilized surface soil (0-30 cm) water, deep soil (80-100 cm) water and groundwater in a much balanced pattern; in July and August, the water source gradually shifted to deep soil water and groundwater, and almost no surface soil water was used; during September and October after the rainy season, the water source gradually shifted to the shallow layer soil water. During growing season, the seasonal fluctuation of soil moisture significantly affected Gs of Q. variabilis. Under good soil water conditions, Gs was higher and WUE is lower; under serious water shortage, Q. variabilis can reduce stomatal conductance to decrease water loss and obtain higher carbon assimilation rate and maintain higher water use efficiency, in order to adapt to the arid condition.