The paper aimed to study the characteristics of stem sap flow, the transpiration rate and their correlations with the environmental factors for tree species, Populus hopeiensis, P. simonii and Armeniaca ansu in the mixed area with both water and wind erosion on the Loess Plateau of northwestern China. We used PT-stem flow and LI-6400 Photosynthesis systems, and a LI-1400 meteorological station. The stem sap flow was usually weak at nighttime. The regulation of single-peak and multiple-peak was not obvious. The start and ending time of sap flow of the three species were similar, starting at 7:00 am and ended at 8:04 pm. The diurnal stem sap flow rate of P. hopeiensis′s, P. simonii′s and A. ansu′s were (3.65±0.40), (2.22±0.21) and (1.63±0.13) kilograms per hour, respectively, resulting in daily water use of (49.29±5.42), (30.64±2.92) and (22.19±1.73) kilograms, respectively. Overcast and rainy weather influenced transpiration sharply. The stem sap flow followed the environmental factors closely. The starting and the ending points of the stem sap flow fell behind the corresponding changes of environmental factors with different lags. The peaks of the stem sap flow came ealier than other environmental factors. The stem sap flow had significant positive correlations with photosynthetic active radiation, air temperature, soil temperature and wind speed, but had significant negative correlation with air relative humidity. The rank in influencing sapflow rate among environmental factors in a descending order was: photosynthetic active radiation, air temperature, air relative humidity, soil temperature and wind speed.