Abstract: Populus euphratica Oliv. sap flow and meteorological factors (i.e., total solar radiation, wind speed, air temperature, air humidity, and vapor pressure deficit) were observed synchronously for five months to reveal the impact of such factors on sap flow at different time scales. Results showed that on the monthly scale, total solar radiation was the main factor influencing sap flow of P. euphratica, explaining 98.3% of sap flow changes. On the daily scale, air temperature, air humidity, wind speed, and total solar radiation had the greatest influence on changes in the sap flow rate, with air temperature explaining 33.0% of sap flow changes, and all four factors explaining 55.2% of sap flow changes. On the hourly scale, total solar radiation had the greatest impact on sap flow in May to September 2018. With the increase in time scale, fewer factors affected the flow rate change. Predicting P. euphratica sap flow on an hourly scale based on meteorological factors required more parameters and showed the least reliability. Predicting the monthly sap flow rate required fewer parameters and showed the maximum reliability. Thus, predicting the sap flow rate of P. euphratica would be more appropriate at the monthly scale.