This study employed the thermal diffusion probe method to observe the sap flow of Populus simonii and Populus alba var. pyramidalis in the Kubuqi Desert from May to October 2021. Meteorological factors and soil moisture were synchronously monitored during the growing season to analyze the response relationship between sap flow and environmental factors (air temperature, relative humidity, vapor pressure deficit, solar radiation, wind speed, soil water content, and precipitation) at hourly, daily, and monthly scales. The results showed that: (1) The total water consumption of P. alba var. pyramidalis (1059.43 kg) was higher than that of P. simonii (947.30 kg). (2) On the hourly scale, the sap flow velocity of P. simonii and P. alba var. pyramidalis were highly significant correlations with seven environmental factors, including air temperature, relative humidity, vapor pressure deficit, solar radiation, wind speed, soil water content, and precipitation. On the daily scale, the sap flow velocity of two species were highly significant correlations with six environmental factors, including air temperature, relative humidity, vapor pressure deficit, solar radiation, wind speed, and soil water content. On the monthly scale, the sap flow velocity of P. simonii was highly and significantly correlated with vapor pressure deficit, solar radiation, and soil water content, while that of P. alba var. pyramidalis was highly and significantly correlated with vapor pressure deficit, solar radiation, and soil water content. (3) The relationship between sap flow and environmental factors was established at different temporal scales. On the hourly scale, the selected factors for P. simonii were air temperature, soil water content, vapor pressure deficit and wind speed, with explanatory rate of 74.9%. For P. alba var. pyramidalis, the selected factors included solar radiation and air temperature, etc., with explanatory rate of 75.6%. On the daily scale, the selected factors for P. simonii were soil water content, air temperature, and wind speed, with explanatory rate of 84.3%. For P. alba var. pyramidalis, the selected factors included solar radiation, air temperature, soil water content, and wind speed, with explanatory rate of 80.9%. On the monthly scale, solar radiation and wind speed were the main influencing factors for the sap flow velocity of P. simonii, jointly explaining 99.5% of the variation. Vapor pressure deficit had the greatest impact on the sap flow velocity of P. alba var. pyramidalis, explaining 91.8% of the variation alone. (4) At smaller temporal scale (hourly), sap flow could be directly measured using instruments. While at larger temporal scales (daily and monthly), environmental factors could be monitored to estimate transpiration water consumption for both tree species.