Abstract: Against the background of global climate change and regional water scarcity, in-depth research on the synergistic driving mechanisms of multiple factors controlling vegetation water use in arid and semi-arid regions is urgently required. Existing studies lack sufficient analysis on the synergistic regulation of meteorology-soil water-groundwater in the sap flow process of Salix matsudana Koidz in the Mu Us Sandy Land. Therefore, taking 40-year-old Salix matsudana Koidz in the Mu Us Sandy Land as the research object, this study systematically analyzed the water use strategy of Salix matsudana Koidz under the synergistic effect of meteorological factors and groundwater in the rainy season, and clarified the key paths of meteorological factors regulating the sap flow process of Salix matsudana Koidz, through in-situ continuous monitoring of meteorological elements, groundwater depth, soil moisture content and stem sap flow density, combined with Structural Equation Model and Boosted Regression Tree model. The results show that Salix matsudana Koidz is a deep water-dependent tree species, forming a water use strategy dynamically adjusted with hydro-meteorological conditions in the growing season. It preferentially utilizes deep soil water and groundwater at 250–450 cm, with extremely low utilization efficiency of surface soil water recharged by rainfall. Meteorological factors regulate the sap flow of Salix matsudana Koidz through dual paths of direct driving and indirect synergy. Solar radiation (β=0.71, P<0.001) is the core direct driving factor, relative humidity (β=−0.27, P<0.05) is the key atmospheric constraint factor, and groundwater level exerts a significant positive indirect effect (total effect 0.39) through the hydrological transmission chain of groundwater-deep soil water-vegetation sap flow. The sap flow density of Salix matsudana Koidz peaks when the air temperature is 9.97–21.20℃ and the relative humidity is moderate. This study clarifies the multi-factor coupled driving mechanism of the sap flow process of Salix matsudana Koidz and provides theoretical support for the management of Salix matsudana Koidz forests and ecological restoration projects in the Mu Us Sandy Land.