Water availability is a key factor that sustains plant growth and determines vegetation distribution pattern. Plant water uptake pattern is regarded as a foundation to reveal the interaction mechanism between hydrological changes and vegetation succession. As the largest freshwater lake in China, the hydrological regime of Lake Poyang has been encountering a dramatic change in terms of water level variations, which generated a large influence on the water recharge of the wetland ecosystem. In this study, the replenishment sources of soil water in the Artemisia capillaris community, a typical mesophyte vegetation in the Lake Poyang wetland, were explored by using the stable isotopes (δ¹⁸O and δD). The stable isotopes composition of rainfall, lake water, groundwater, soil water and the plant water were analyzed. The main water uptake depth and the proportions of water uptake by A. capillaris were further investigated, using a direct inference method and the IsoSource mixing model. The results showed that the lake water and soil water isotopes were enriched, while the groundwater isotopes were rarely fractionated, relative to local precipitation isotopes. The wetland groundwater is likely to be recharged by the long-term rainfall and the lake water. During the rainy season (April-June), soil water in the A. capillaris community was mainly recharged by the local rainfall. During the summer months (July-August), the deep soil water was most likely to be supplied by lateral lake water and the groundwater. Accordingly, the soil water moved upward from deep to shallow soil layers under strong evaporation. In the growing season, A. capillaris mainly absorbed soil water at 0-80 cm depths and showed plasticity in switching water sources from different soil layers. When the soil water content was high (April-May), A. capillaris mainly used water from shallow soil layers (0-40 cm), accounting for 49%-68% of the total water uptake. When the shallow soil becomes dry (June-August), A. capillaris mainly absorbed deep soil water from deep soil layers (40-80 cm). The proportions of soil water uptake at 40-80 cm were up to 74%-95%. However, the main water source of A. capillaris again switched to the soil water at top soil layer (0-15 cm) during the mature stage (September-October), and the proportions of water uptake were in the range of 41%-70%. Overall, these findings reveal that lake water is an important replenishment source of the soil water in mesophyte vegetation community in the Lake Poyang wetland. A. capillaris could adjust water uptake depth according to the soil water availability, suggesting a strong drought tolerance. This study is helpful to provide scientific reference for the ecosystem evolution and wetland protection in the Lake Poyang-floodplains.