Tree transpiration plays a determining role in water balance for forest stands and in water yield from forested catchments. In the present study, an experiment was conducted in the Pailugou watershed in the Qilian Mountains, in the arid region of Northwest China. In a 86-year-old Picea crassifolia forest stand, 3-5 trees from each of dominant, subdominant, intermediate and suppressed trees were chosen as sample trees. The sap flux density for these trees was measured using the thermal dissipation probe (TDP) method from June 16 to October 14, 2015. Furthermore, the related meteorological factors, including solar radiation (R_s), air temperature (T), air relative humidity (R_h), and precipitation (P) were simultaneously monitored by an automatic meteorological station in the outer forest. The results showed that: (1) Daily sap flow density was higher during the day than during the night. On sunny days, daily variation in sap flow density exhibited large amplitudes, whereas it exhibited little variation on cloudy and rainy days. (2) On sunny days, daily sap flow density began earlier in the morning, ended later at night, and exhibited a larger maximum of sap flow density as tree dominance class increased. The value of the mean sap flow density of dominant trees was (0.0758 ± 0.0475) mL cm^-2 min^-1, which was 1.5 times of that of subdominant trees and 1.68 times that of intermediate and suppressed trees. (3) Mean daily sap flow density for all dominant trees showed that the maximum values were achieved in June, with the next highest values in August, and decreasing values in September and October. In addition, sap flow density exhibited the following order: dominant > subdominant > intermediate > suppressed trees. (4) Correlation and stepwise regression analyses indicated that mean daily sap flow density was significantly correlated with solar radiation, vapor pressure deficit, and air temperature, and was negatively correlated with air relative humidity and precipitation. Solar radiation was the main meteorological factor influencing mean daily sap flow density of dominant, subdominant, and intermediate trees, whereas air relative humidity primarily influenced suppressed trees.