Sap flux density of Ulmus pumila var. sabulosa in typical elms sparse forest in Hunshandake Sandland of Inner Mongolia was studied by using Granier technology. Nine dominant trees with different Diameters at Breast Height (DBH) which belong to three classes of DBH, that is, 15—25cm (small DBH), 25—35cm (medium DBH) and >35cm (large DBH), respectively in natural forest were selected and measured in terms of sap flux density. At the same time, various environmental factors including air temperature, relative humidity, photosynthetically active radiation, and wind speed were monitored in the whole growing season. The sap flow characteristics of forest stand are: sap flow density peak value of large DBH is larger than that of medium DBH tree and then small DBH tree. Sap flow density of large DBH trees change gently because age of stand. Average monthly sap flow density is less with the shift of growing season; the sap flow density enters into the state of weakly fluctuation in the end of growing season (October). The maximum value of average monthly sap flow density for varied tree species appears from June to August, the differences among dominant, medium and small trees are large. The main influence environmental factors are VPD, PAR, Ta. At different months, the dominant environmental factors have changed, but the two main factors are PAR and VPD.