Abstract: The stem sap method is the main method to determine tree transpiration internationally, but the sap flow of tree species in evergreen broadleaved forest has not been studied yet. Especially it is not clear in the relation between velocities of stem sap flow and environmental factors. Therefore, we employ sap flow sensor to monitor sap flow of Castanopsis fargesi in evergreen broadleaved forest of Dagangshan Mountain. Meanwhile, an Automatic Weather Station was applied to measure meteorological parameters and soil factors. The results showed that the stern sap flow behaved as double peaks curve. Among 28 environmental factors, wind velocity, soil temperature at 10cm underground, air temperature at 20m above ground, relative humidity at 20m above ground and soil water potential are the major factors affecting sap flow rate. The next factors are solar radiation intensity, Photosynthetic Active Radiation（PAR） and saturation vapor pressure. Above all, the dominant factors were soil temperature and air temperature. The conclusion has momentous significance to accurately evaluate the effect of forest in water circle of subtropical zone.