Abstract: Hydrological conditions strongly influence plant water use and vegetation distribution in floodplain wetlands. Investigating plant water use strategies and their responses to groundwater depth (GWD) variation is essential for understanding wetland vegetation changes. This study used stable isotopes (δ¹⁸O, δD, and δ¹³C) and the MixSIAR (Mixture Stable Isotope Analysis in R) model to examine water use characteristics and water use efficiency (WUE) among different dominant plants across an elevation gradient in Poyang Lake wetland, China. The results showed a trend of increasing groundwater reliance but decreasing WUE as GWD declined. At the high-elevation site with deep GWD (avg. (6.47 ± 2.09) m), Artemisia capillaris primarily extracted shallow and middle soil water (58.2%–69.3%) in wet spring, but shifted to deep soil water (48.6%–60.2%) in summer when soil water content (SWC) decreased. To adapt to water deficit, it presented a significant increase in WUE during dry summer and autumn (P < 0.05). The δ¹³C of A. capillaris exhibited a significant (P < 0.01) negative correlation with SWC, indicating that it adopts a conservative water use strategy. At the mid-elevation site with median GWD (avg. (4.23 ± 2.03) m), the water use ratios of groundwater and deep soil water for Phragmites australis showed significant negative linear correlations with GWD (P < 0.05). As GWD increased in dry autumn, P. australis switched from primarily using groundwater to absorbing shallow and middle soil water, and significantly improved WUE (P < 0.05), showing a flexible water use strategy. In contrast, at the low-elevation site under the shallowest GWD (avg. 2.50±1.78 m), Carex cinerascens mainly relied on groundwater and deep soil water (56.1%–69.2%) throughout late spring and autumn. It maintained the lowest and most stable WUE among the three species, indicating a profligate water use strategy. The findings highlight that adjusting water use patterns and improving WUE are crucial to enhancing species adaptability to changing hydrological conditions. This work improves our understanding of the responses of plant water uptake strategies to hydrological variations and the mechanisms driving vegetation succession in the Poyang Lake wetland.