The loess hilly region is located in an ecologically fragile area with seasonal drought. Clarifying the impact of changes in the water environment on the water source of regional dominant afforestation tree species is of great significance to the water use of plants and their symbiosis in arid areas. Therefore, two kinds of mixed forests (Hippophae rhamnoides + Pinus tabuliformis, H_tP_t, H. rhamnoides+Robinia pseudoacacia, H_tR_p), the pure forests of H. rhamnoides (H_t), P. tabuliformis (P_t), and R. pseudoacacia (R_p) were selected. The isotope composition of xylem water, soil water and precipitation were determined. The water use and the relationship among symbiotic plants were quantitatively analyzed by using the IsoSource model and the proportional similarity (PS) index. The results indicated that:(1) during the dry season, there were differences in water absorption layer between pure forest and mixed forest of the same tree species. The tree species in the pure forest mainly used soil water in the 0-100 cm. The utilization ratios of H_r and R_p to 40-100 cm were 46.8% and 43.8%, and P_t from 0-40 cm was 83.9%. While, in the mixed forests were more inclined to use 40-200 cm. Among them, the utilization rate of H_rP_t (H. rhamnoides) from 100-200 cm layer was 58.8%, H_rP_t (P. tabuliformis) from 40-100 cm was 47.4%. H_rR_p (H. rhamnoides) and H_rR_p (R. pseudoacacia) from 100-200 cm were 58.2% and 65.3%, respectively. (2) During the wet season, tree species in pure and mixed forests were mainly use soil water in the 0-100 cm, and the utilization rates was above 70%. (3) The PS index in the dry season was significantly greater than that in the wet season, and in the H_rR_p was the highest, indicating that there might be water competition between H. rhamnoides and R. pseudoacacia. In summary, compared with H_rR_p, symbiotic plants in H_rP_t mainly respond to seasonal droughts through spatial differences in water use, which made it much easier for this ecosystem to be adaptive to the fragile ecological environment in the region. The research results can provide a basis for the restoration and reconstruction of regional degraded ecology and the construction of drought-adaptive measures system.