Afforestation is an important measure for ecological restoration. Due to the high planting density, the soil becomes dry, the growth of trees declines, and the improvement of ecological function is restricted seriously. Thinning is an important means of the artificial forest reconstruction, and water use characteristic is the key factor to determine suitable thinning density. However, there is few systematic studies about the effects of thinning on water use characteristics. This study set up four treatments including thinning 55% (P1), 28% (P2), 16% (P3) and control (P4) for Robinia pseudoacacia plantation in the loess hilly region. The δ²H and δ¹⁸O values in soil water and stem water at 0-500 cm depth and leaf δ¹³C value were measured. The differences of water source and water use efficiency of R. pseudoacacia among different thinning intensities were compared. The quantitative relationships between water source ratio and δ¹³C were established. The results showed that the soil water content of the thinned plots at different depths was higher than that of the control plot, which indicated that thinning played an important role in alleviating soil water stress. The contribution proportion of soil water in shallow (0-100 cm) and middle layers (100-300 cm) to R. pseudoacacia increased with the thinning intensity (P1:80.4%; P2:78.1%; P3:76.3%; P4:67.8%), and the contribution proportion of soil water in deep layer (300-500 cm) decreased relatively. The leaf δ¹³C of the thinned plots was higher than that of the control plot, which indicated that thinning improved water use efficiency of R. pseudoacacia. There was a positive linear correlation between δ¹³C value of R. pseudoacacia and the contribution proportion of soil water in shallow layer (P<0.01), while a negative linear correlation was found between δ¹³C value and the contribution proportion of soil water in the middle and deep layers (P<0.05). These findings gave a preliminary understanding about the effects of thinning on the water use characteristics of R. pseudoacacia plantation, and provided the scientific basis for the reconstruction of artificial R. pseudoacacia plantations and the improvement of ecological function in the Loess Plateau. It is necessary to carry out long-term observation after thinning in the future.