Based on the 12 months observation of the sap flow rate and environmental factors by using sap flow meter and mini weather station in Dongshanqiao Forest of Nanjing City from September 2012 to August 2013, the impact factors of the sap flow of Chinese fir sapling and its contribution to transpiration of Chinese fir forest was analyzed. Results indicated that:(1) The sap flow rate of Chinese fir sapling showed seasonal differences and the change of the sap flow rate took on a single-peak curve in different seasons. Sap flow rates in daytime were higher than those during the night.(2) The crest values of the sap flow rate in spring, summer, autumn and winter was 79.97, 105.22, 70.30, 33.19 g/h, respectively.(3) The domenant environmental factor of sap flow rate was solar radiation, the next was vapor pressure deficiency. The correlation between daily sap flow and solar radiation, air temperature, vapor pressure deficiency, soil temperature at all layers showed a significant positive correlation.(4) The models to relate the sap flow rate, daily sap flow with environmental factors were established. These models could better explain the change of the sap flow rate and daily sap flow.(5) The ratio of contribution of Chinese fir saplings sap flow throughout the year to the total transpiration of Chinese fir forest was 6.12%, and the ratio of contribution in summer was the largest.