Water is a key factor limiting plant growth; plant water uptake sources reflect the ecological hydrological processes and vegetation responses to environmental changes, thus becoming an important focus in vegetation construction. However, the water use patterns of the typical vegetation in the Qinling Mountains remain unclear. This study focused on the main afforestation species in the Qinling Mountains, including Quercus aliena var. acutiserrata, Quercus acutissima Carr. and Pinus massoniana Lamb. By analyzing δ²H and δ¹⁸O isotopes in precipitation, soil water, and xylem water, we employed the MixSIAR model to quantitatively assess water utilization characteristics, and calculated niche width and overlap using the Levins index and proportion similarity index. Results showed that the plants had varying water use sources during the same period, and the same tree species also had varying water sources before and after precipitation. Quercus aliena var. acutiserrata mainly took water from the 20-40 cm soil layer (42.9%) before the rainfall, yet transitioned to the shallower 0-20 cm layer after precipitation (56.3%-58.2%). Before the rain, Quercus acutissima Carr. primarily utilized soil water from the deep layers of 40-60 cm (45.3%), which gradually shifted to shallow soil layer after the rain, and the proportion increased from 40% to 58.7% with the increase of rainfall. The water use sources of Pinus massoniana Lamb exhibited no significant alterations before and after minor rainfall events; however, following heavy rainfall (259 mm), the contribution of soil water from the shallow 0-20 cm layer rose from 38% to 91.1%. The PS index of Quercus aliena var. acutiserrata and Quercus acutissima Carr. forest was 77.3%, indicating weak water competition between the two plant species. However, there was a clear water competition relationship between Quercus aliena var. acutiserrata and Pinus massoniana Lamb. forests, Quercus acutissima Carr. and Pinus massoniana Lamb. forest (PS index>80%). In conclusion, the mixed forests comprising Quercus aliena var. acutiserrata and Quercus acutissima Carr. represent a suitable combination of tree species, demonstrating relatively weak competition for water within the same soil layer. However, the water use strategies of Quercus acutissima Carr. were similar to Pinus massoniana Lamb before and after the precipitation, showing competition for water resources. Therefore, in order to maintain sustainable forest management, it is suggested to choose a reasonable tree species configuration in future vegetation construction.