Studies on the relationship between sap flow activities and major environmental factors have mainly focused on solar radiation; air temperature, air moisture, soil moisture and vapor pressure deficit, however, the effect of soil temperature were usually neglected. In fact, soil water may not be available to trees and thus leading to the potential loss of forest yield due to reduction of transpiration and photosynthesis by low soil temperature. Therefore quantified studies on the influence of soil temperature on sap flow are necessary. Accordingly, the sap flow velocity of Pinus tabulaeformis, soil temperature and other environmental factors were synchronously measured in one year with Thermal Dissipation Probe systems (TDP-30) and an automatic weather station. The results show that the soil layers in which soil temperature has strong effect on sap flow velocity are related with root distributions, and soil temperature conditions at which water could be easier absorbed. Soil temperature of surface layer has the most significant influence on sap flow activities in spring while that of deep layer in summer and middle layer in autumn. Furthermore, the effect of soil temperature on sap flow is most significant at 10.0-14.9℃, and the soil temperature threshold is about 10℃.