The Tibetan Plateau, a pivotal component of the global climate system known as the "Asian Water Tower," governs freshwater availability for billions. However, the physical mechanisms linking stratospheric circulation to its precipitation variability remain poorly constrained, limiting predictive understanding. Here, this work constructs a new indicator based on the ratio of stratospheric tracer ¹⁰Be (t_1/2 = 1.39 Ma) and ⁷Be (t_1/2 = 53.29 d), to reveal the modulation mechanism of stratospheric Quasi-Biennial Oscillation (QBO) phase transitions on Tibetan Plateau precipitation processes and its possible large-scale vertical circulation associations. Analyzing synchronous wet-deposition data from Lhasa (Tibetan Plateau) and Xi'an (Loess Plateau) during the 2022–2023 QBO transition, we empirically analyzes the synchronous response relationship between isotope deposition and regional precipitation during the tropopause stable period determined by the ¹⁰Be/⁷Be ratio in precipitation samples. An XGBoost machine-learning model then isolates the coupled impact of the easterly QBO phase and upper-level circulation on precipitation. Our results demonstrate that during the observation period, the easterly QBO excites a meridional wind dipole, driving an anticyclonic circulation that enhances stratospheric air transport to the surface. This dynamical pathway substantially increases precipitation in the southern Tibetan Plateau by approximately 31 %. Attempting to mechanistically linking a fundamental mode of global atmospheric variability to regional water resources via stratospheric isotopic evidence, this framework advances the understanding of cross-scale interactions within the Earth system, with direct implications for evaluating climate model performance and future water security under global change.