Abstract: Stem sap flow rising through the xylem is an important physiological index to measure water use and transport of plants, and it can better reflect the plants transpiration and water metabolism process. Pruning buds after planting is a kind of economic and effective tending measure widely used in crops, economic forest, and timber trees. It can avoid a large number of lateral buds germination, promote plant growth, and improve the yield and quality of target products. The purpose of this study was to investigate the effects of bud pruning intensity on sap flow velocity characteristics and its main environmental factors of Populus tomentosa. Four treatments of bud pruning were designed. After afforestation in 2016, the buds below 1/3 (1.0 m, BP1.0, mild), 1/2 (1.5 m, BP1.5, moderate) and 2/3 (2.0 m, BP2.0, severe) of tree height were removed respectively. A control treatment (CK) without removing any buds was also designed. The sap flow velocity of P.tomentosa and environmental factors with different bud pruning intensities were continuously determined from August 2017 to June 2018 by using thermal diffusion flow detection technology and automatic weather station. The results showed that: 1) The diurnal dynamics of sap flow velocity showed a single peak curve in sunny days, which increased rapidly from 5:30 to 9:40, reached the peak at 11:00 to 14:40 and then decreased gradually, and began to run at low value at night from 16:30 to 22:00. The shape of diurnal dynamic curve of sap flow was not affected by the bud pruning intensities. 2) The effect of bud pruning intensity on peak value, daily average value and other characteristic values of sap flow velocity changed with time. No significant difference was found in the characteristic values of each treatment in the early stage of the experiment (August and September 2017). As time went on, the maximum values of daily and monthly average of sap flow velocity shifted from CK to BP1.0. The mean value of sap flow velocity was always the smallest under severe bud pruning (BP2.0). 3) The effect of bud pruning on average sap flow velocity was significant (P<0.01). The average sap flow velocity values of CK, BP1.0, BP1.5, BP2.0 were 1.405×10-3, 1.503×10-3, 1.462×10-3, 1.033×10-3 cm/s, respectively from August 2017 to June 2018. And the sap flow velocity of BP2.0 was significantly lower than that of the other treatments, but there was no significant difference among CK, BP1.0 and BP1.5 (P>0.05). 4) The main environmental factors affecting sap flow velocity in different periods and treatments were different. Air temperature and radiation were the main factors affecting the instantaneous value during daytime, while vapor pressure deficit, radiation and air humidity were the main factors affecting the instantaneous value at night. However, for the mean sap flow velocity, radiation and maximum air temperature were the main drivers of daytime mean values, and minimum air temperature and air humidity were the main drivers of nighttime mean values. 5) The nighttime sap flow velocity of young P.tomentosa accounted for 5%-70% of the whole day sap flow velocity under different bud pruning intensities. The nighttime sap flow velocity of different treatments showed significant differences from the third growing season after afforestation. To sum up, bud pruning did not affect the diurnal process of sap flow, but affected the pattern of sap flow and the main environmental factors. With the increase of bud pruning intensity, the smaller average daily sap flow velocity would result in the greater proportion of sap flow velocity at night. From the point of view of promoting plant water physiological activities, the light and moderate bud pruning methods are more suitable.