Water is very important ecological factor for the structure and function of plant communities in arid and semiarid regions, including coastal zones. Determination of water use patterns of plants in the coastal zones is critical to our understanding of local eco-hydrological processes and predicting the trends in the ecological succession under global climate change. To determine the water use patterns of Tamarix chinensis under different water conditions of Shell Islands in the Yellow River Delta, we analyzed the moisture and salinity in soils and delta~(18)O values of different potential water sources and the water in the xylem of Tamarix chinensis. The IsoSource model was employed to estimate the contributions of potential water sources to the water in the xylem of Tamarix chinensis. On July 20-22, 2013, there was abundant precipitation, 78.2% of the water used by Tamarix chinensis was from the water in the soil (30.3% from the shallow soil (0-60 cm depth) and 47.9% from deep soil (60-100 cm depth). On June 21-23, 2014, there was less precipitation, the main water source of Tamarix chinensis shifted from the water in the soil to shallow groundwater as the water in the soil decreased significantly. Specifically, Tamarix chinensis obtained the water of 79.1% and 20.9% from shallow groundwater and the water in the soil, respectively, under these conditions. Tamarix chinensis in the Shell Islands in the Yellow River Delta could improve its drought resistance by shifting its main water sources, resulting in competitive and increased water use efficiency.