By means of artificial control four gradients of soil water content (SWC) were established under natural condition, transpiration rate(Tr) and net photosynthesis rate(Pn) and environmental factors of Oriental Arborvitae were measured in runoff forestry research station at Fangshan county of Shanxi province from April to October in 2002. The results show that Tr has a close relation with SWC. It significantly decreases with decline of SWC, and the severer soil water stress, the earlier time of peak of Pn. As the SWC do not induces soil water stress, the diurnal changes of Tr accord with the diurnal change of radiation intensity and temperature. Under the situation of this research, the water stress of Oriental Arborviate appeared as the SWC was below 5.79%, and it became smaller when the SWC approached to 6.50% in spring, while it was very severe below 6.94% SWC in summer, no water stress was observed in autumn but Tr still changed according to SWC. This result provides evidence for measurements of runoff forestry and its effective management on the soil moisture.