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Water amount stored in sapwood and its participation in sap flow cycle are considered to be one of the important mechanisms for trees忆 drought resistance. Based on monitoring the sap flux of Populus bolleana at crown base and stem base with thermal dissipation probe technique (TDP) , combined with synchronous meteorological observation, this paper analyzed the relationship between sap flow fluxes and atmospheric evaporation potential (ET 0) at two positions. Results showed that sap flow flux at crown base (Q u) was a reliable indicator for estimating sap flux of P. bolleana individuals. The average daily Q u in typical sunny days from June to September was (7. 61 依0. 65) L / d, which was higher than those in cloudy and overcast days by 0. 41 and 2. 71 L / d respectively. The sap flow flux in stem base (Q d) , varied greatly in different seasons, was lower than Q u during the sunny days from June to August but reversed rapidly in late August following continuous rainfall, by which P. bolleana maintained water鄄balance and solved water deficit partly. After continued rainfall in late August, soil moisture improved effectively, so the percentage of nocturnal sap flux to total daily Q d reduced from 31郾 98-35. 36% to 6. 72%-7. 99% , by which water deficit could be compensated effectively. P. bolleana have experienced water supplement (in June) , loss (in July and August) and re鄄supplement (in September) processes during