[Objective] In order to understand the water use strategy of Ailanthus altissima and to formulate efficient water management measures for A. altissima, we investigated the characteristics of sap flow and radial stem variation of A. altissima under four volumetric soil moisture content treatments[10%~15% (W₁), 15%~20% (W₂), 20%~25% (W₃), and 25%~30% (W₄)] and their relationships with meteorological factors.[Methods] Thermal diffusion probes (TDP) were used to continuously observe sap flow, and radial stem change recorders were used to monitor changes in stem diameter of A. altissima. Changes in related environmental factors were simultaneously monitored.[Results] Sap flow was almost the same, and growth rate and variation of stem diameter were similar for the W₃ and W₄ treatments. Sap flow increased with increasing soil moisture in the range of W₁ to W₃, whereas the proportion of nighttime sap flow to total daily sap flow showed the opposite trend[W₁ (12.3%)>W₂ (11.9%)>W₃ (6.0%)]. Variations in the stem diameter of A. altissima at the daily scale decreased with increasing soil moisture, but the growth rate increased accordingly. The correlation between sap flow and meteorological factors increased with increasing soil moisture content during the daytime, but decreased at night. Changes in both sap flow and stem diameter lagged behind solar radiation and ahead of vapor pressure deficit (VPD), and the time lag between these two indicators and VPD decreased with increasing soil moisture, while the time lag between both and solar radiation was little affected. Changes in sap flow and stem diameter of A. altissima at the daily scale showed an inverse change pattern, and the changes in sap flow always preceded the changes in stem diameter, and the time lag shortened as soil moisture increased.[Conclusion] Changes in sap flow and stem diameter of A. altissima to soil moisture changes were closely related. The soil moisture threshold value at which A. altissima could fully utilize soil moisture was the W₃ level. Therefore, the W₃ soil moisture treatment is the most suitable irrigation threshold for A. altissima.