In this paper, the heat pulse technique was applied to study the stem sap flow of Populus euphratica in the desert riparian forest in extreme arid region from April to October of 2003 and from May to October of 2004. The experimental sites are located at Qidaoqiao(41°59′ N,101°10′ E) and Bayantaolai farm(42°01′ N,101°14′ E) in Ejin county, in the low reach of Heihe river basin. The heat-pulse velocity (HPV) method was used to characterize the spatial and seasonal transpiration variability of Populus euphratica. Estimates of whole-tree transpiration trend to differ by less than 15% from HPV sensors and cut-tree (photometer) measurements. The aim of this study was: ①to measure transpiration of Populus euphratica, and to establish diurnal, monthly, and annual transpiration velocity of Populus euphratica; ②to characterize spatial and seasonal transpiration variability of Populus euphratica in desert riparian forest in extreme arid region; ③to study stem sap flow at different age of Populus euphratica; ④to study instantaneous responses of tree water flux to the microclimatic factors, and to establish the statistical model between microclimatic factors and sap flow flux.The results showed that, Populus euphratica is a fast-transpiring tree species with high daily water consumption, in the natural conditions, its sap flow flux density was 0.14~0.29 L\5cm-2\5d-1. The diurnal change of sap flow velocity showed minor fluctuations. At night, the ascent of sap flow could be observed in the above main tree species owning to the existence of root pressure. During the growing season, the maximum of average velocity was observed in July, the second in June and August, that in September was higher than that in May, the minimum occurred in October. From July to August, more than 70 percent occupied the average transpiration during the growing season. The variation of sap flow velocity at different age of Populus euphratica showed that 15 a＞25 a＞50 a.The velocity of sap flow was closely related to the change of micrometeorological factors, the average velocity of sap flow showed prominent linear correlation with net radiation, air temperature, and relative humidity.