Abstract: Transpiration characteristic of Ailanthus altissima,major tree for forestation in the Loess Plateau was analyzed.Physiological indices of Ailanthus altissima in different soil water conditions were measured by portable steady porometer(LI-1600) during the period of May to October of 2006,in Fangshan, Shanxi Province(Forestry Experimental Station of Beijing Forestry University). The response processes of physiological absorbed radiation,stomatal resistance,transpiration rate to different soil water content were studied.The results showed,the relationship between transpiration rate and physiological absorbed radiation of Ailanthus altissima in semi-arid regions of Loess Plateau was power function.Meanwhile,the relationship between transpiration rate and stomatal resistance and soil water content was close.Generally,transpiration rate increased with the raising of irradiance and soil water content.The correlation coefficient between transpiration rate and soil water content was in average coming up to 0.891 7(R²=0.891 7).Based on the above analysis of the transpiration rate in different soil water conditions,therefore,to lighten injuring on physiology as transpiration seriously,to increase the rate of survivor and to promote growing,it is necessary to adopt measure as avoiding violent irradiance of the sapling when the soil is short of water.The soil water content during 15% to 20% is beneficial to increase the water use efficiency of plants,which could help plants to enhance water use efficiency in arid and semiarid area.