Abstract: The aim of this study was to investigate variation of water utilization and ecological niche characteristics of tree species (Robinia pseudoacacia, Koelreuteria paniculata, and Platycladus orientalis) and understory shrubs (Vitex negundo) in artificial pure forests during the growing seasons of 2023 (June-October) and 2024 (May-September) in the rocky mountainous areas of Beijing. The water sources of the tree species and the understory shrub were analyzed using hydrogen and oxygen stable isotope techniques and the MixSIAR model. Ecological niche characteristics were calculated using the Levine index and the Levins overlap index. The results showed that: (1) During the early dry season (May–June), R. pseudoacacia depended primarily on groundwater, K. paniculata utilized 60–80 cm soil water and groundwater, while P. orientalis relied on 0–40 cm shallow soil water. In the rainy season (July–August), three tree species shifted to 0–40 cm shallow soil water. By the late dry season (September–October), R. pseudoacacia and K. paniculata primarily exploited 40–80 cm deep soil water, whereas P. orientalis continued to utilize 0–40 cm shallow soil water. Understory shrubs in all forest stands absorbed 0–40 cm shallow soil water during the dry season and 0–20 cm soil water during the rainy season. (2) In the early dry season, P. orientalis and understory shrubs exhibited high water niche overlap, indicating potential competition. During the rainy season, water niche overlap between R. pseudoacacia, K. paniculata, P. orientalis, and understory shrubs increased, but sufficient soil moisture mitigated water stress. In summary, tree species adapted to seasonal drought by dynamically adjusting the depth of water absorption (groundwater or deep water), while understory shrubs relied on shallow soil water. The spatiotemporal differentiation of their ecological niches reduced soil water competition and enhanced ecosystem stability. These findings could provide theoretical guidance for vegetation restoration and management.