Soil water plays a crucial role in the hydrological cycle by, for example, delivering nutrients to plants, acting as storage to buffer deficiencies in surface water availability, and serving as a medium to transfer suspended and dissolved material. Understanding water balance processes can be aided by knowledge of the relative contributions of soil water source components, which could be a mix of recent precipitation, older groundwater, and water that has been subjected to evapotranspiration. We used stable oxygen isotopes as a geochemical tracer to identify soil water composition in a wetland area of Poyang Lake, the largest freshwater lake in China, which contains the largest single area of habitat for migratory birds. The focus was on characterizing the variation with depth of the stable isotope composition in soil water profiles, understanding the mechanisms of soil water movement, and identifying the main source of soil water recharge. Sampling was undertaken from July to September 2013 in Poyang Lake National Nature Protection Area at three wetland sites with different land cover characteristics and different degrees of proximity to the lake and river environments. Water samples collected from the Xiu and Gan rivers, which flow into the study area, and soil water sampled at 10 cm intervals over the top 200 cm, were analyzed for stable oxygen isotope composition. For all soil water samples, the value of δD relative to δ¹⁸O plotted above the global meteoric water line (GMWL), implying an evaporation level lower than the global average. The δ¹⁸O value of soil water from all samples ranged from -10.63‰ to -1.17‰, with mean values of -3.99‰, -4.99‰, and -5.02‰ for the three sites. The mean δ¹⁸O value of soil water was lowest in July. The stable isotope compositions of soil water were found to be mainly controlled by precipitation and evaporation. The value of δ¹⁸O declined with increasing soil depth, with most values of δ¹⁸O at depths of 10 to 60 cm ranging from -6‰ to -2‰, at depths of 80 to 150 cm ranging from -8‰ to -3‰, and at a depth of 200 cm ranging from -7‰ to -4‰. The ¹⁸O-enrichment of shallow (< 60 cm) soil water was probably due to evaporation, while the stable isotope composition of water in deep soil layers suggested it was a mix of recent precipitation and older groundwater. For shallow (< 80 cm) soil water, the three sites had a similar pattern of stable isotope composition with depth, but there were differences among sites in absolute values. At one site, stable isotope compositions of soil water were mainly influenced by precipitation; at another site, by precipitation and Gan River water; and at the third site, by precipitation and Poyang Lake water. This study provided reference data that could make a useful contribution to the assessment of regional groundwater resources in Poyang Lake.