In Mu Us Sandy Land, Sabina vulgaris, Artemisia ordosica, and Salix psammophila are the three dominant shrub communities. The water source of the three dominant shrubs and the associated shrub Hedysarum laeve were investigated by stable isotope technology, and leaf stable carbon isotope composition and soil water dynamics were monitored to clarify how these shrub communities utilize water. The results showed that the δ¹⁸O of shallow soil water in shrub communities was similar to that of rainwater in July and September. S. vulgaris mainly used shallow soil water (25 cm) in May, and mainly used both shallow (10-25 cm) and deep (100-200 cm) soil water in July and September. A. ordosica and its associated plant H. laeve mainly used surface soil water (10 cm) in May, but used surface soil water and deep soil water in July (10 cm and 150 cm) and water from the total soil profile (10-150 cm) in September. S. psammophila mainly used shallow soil water (10-25 cm) in May, where as its associated plant H. laeve mainly used deep soil water (50-200 cm); in contrast, both used shallow and deep soil water in July (10-25 cm and 100-200 cm) and in September (25-200 cm). There were seasonal and inter-specific variances among leaf δ¹³C value of the four plants. The evergreen shrub S. vulgaris had a constant leaf δ¹³C value, which was higher than for the three deciduous shrubs. The leaf δ¹³C value of three deciduous plants was higher in May when the soil water content was lower at a shallow depth. Therefore, shallow soil water that was recharged by rainwater was the main water source for three shrub communities. Based on the availability of soil water, the three shrub communities utilized different depths of soil water in different seasons. There was water competition between H. laeve and A. ordosica or S. psammophila. The evergreen shrub S. vulgaris had a higher leaf stable carbon isotope composition, which may confer a competitive advantage. Deciduous shrubs or semi-shrubs may adapt to the environment by enhancing leaf δ¹³C value during drought.