In order to understand water source of the dominant tree species Platycladus orientalis and its eco-physiological adaptation during growing season in Beijing mountainous area, the Platycladus orientalis of water use strategy under different soil moisture conditions during growing season was studied. We employed the Iso-source software and measured the δD and δ¹⁸O in branches of trees, soil and groundwater to clarify their water source during growing season, and analysis combined with eco-physiological factors,such as leaf water potential (ψ), stomatal conductance (GS) and water use efficiency (WUE). Results show that Platycladus orientalis water use strategy with seasonal fluctuations, the in different seasons of different depths of soil moisture utilization ratio are different. In the dry season, Platycladus orientalis absorbed water in 0-20cm soil layer, the average utilization rate between 27.6-31.3%, at the same time, it utilization of 80-100 cm deep soil water and groundwater are in the range of 27.0%-33.7% and 22.0%-28.9% respectively. In the rainy season, the ratio of Platycladus orientalis absorbed 0-20cm soil water increased range of 47.2%-60.9%, the ratio of absorbed water in 80-100 cm deep soil layer and groundwater is decreased, range of 10.5%-16.2% and 15.2%-19.8% respectively. In dry season, the drought and water shortage of Platycladus orientalis is more serious, leaf water potential 44.5% less than in the rainy season; the peak of Platycladus orientalis stomatal conductance appeared at 10:00 am in the dry season, but 51.67% less than in rainy season, the photosynthetic rate and transpiration rate of peak also corresponding fell 22.3% and 37.0% respectively. Platycladus orientalis can reduce stomatal conductance decrease water loss to obtain higher carbon assimilation rate and maintain higher water use efficiency to adapt the arid condition, showed high adaptability to arid condition.