Abstract: In this paper,TDP sap flow meter was used for studying the nighttime sap flow characteristics of Ulmus pumila and Platycladus orientalis in semi-arid region of Loess Plateau.The results showed that the nighttime sap flux density was much smaller than which in daytime.The major duration of water recharging was from sunset to midnight.By the effects of drought,the amount of nighttime water recharge in Ulmus pumila at rainy days should be much larger than the amount at sunny days;the contribution of nighttime water recharge to total transpiration of Ulmus pumila had significant correlations with environmental factors and the main influencing factors was rainfall;Due to the strong water-retention capacity and drought resistance of Platycladus orientalis,the contribution of nighttime water recharge to total transpiration was not subject to the impact of environmental factors.Compared with Ulmus pumila,Platycladus orientalis showed stronger adjustability in the semi-arid area of Loess Plateau.