By the method of Granier’s thermal dissipation probe, the sap flow density of two exotic tree species (Acacia auriculaeformis and Eucalyptus citriodora) in South China was continuously measured in 2013, and the environmental factors including photosynthetically active radiation (PAR), soil water content (SWC), air temperature (T) and air relative humidity (RH) were measured simultaneously, aiming to characterize the seasonal variations of water use in the two tree species. Obvious diurnal variation patterns of sap flow for both species were observed in any season, yet the average and maximum flow rates were consistently higher in A. auriculaeformis than in E. citriodora. Crosscorrelation analysis showed that the time lag was correlated with tree species and seasons, and sap flow rates were significantly correlated with PAR, T, RH and water vapor pressure deficit (VPD). PAR and VPD were the dominant factors in winter and summer, respectively. With similar sapwood area and diameter in breast height, water consumption of A. auriculaeformis was significantly higher than that of E. citriodora. Seasonal variation of transpiration in A. auriculaeformis was more significant than that in E. citriodora. Sapwood depth in E. citriodora decreased rapidly with age, which resulted in its lower transpiration rate. Thus, the trees of E. citriodora planted in the mid1980s senesced earlier than their peers of A. auriculaeformis.