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The stable isotope composition of precipitation has been widely applied to trace moisture sources. In the westerly dominated areas of central Asia, the isotopic composition of precipitation significantly correlates with surface air temperature, and the temperature effect is widely accepted. However, the significance of isotopes in tracing water vapor source is not clear for this region, and a further investigation using event-based observations is still needed. Based on a network around the Tianshan Mountains, the westerly regime of precipitation isotopes is investigated. Although the seasonality of precipitation isotopes for the northern and southern basins of the Tianshan Mountains is similar, the more arid climate and leeward landform of the southern basin lead to weak moisture transport and relatively enriched oxygen isotopes. Of the synoptic systems resulting in heavy precipitation, the central Asian vortex more commonly leads to depleted isotopic ratios in precipitation, which is related to the contribution of northerly moisture and/or surface moisture. The specific humidity-adjusted Lagrangian diagnostic indicates that the heavy precipitation on the southern basin stations usually has relatively local moisture sources in this arid climate and that terrestrial moisture evaporated from Europe and central Asia, rather than marine moisture from the Atlantic Ocean, is more likely to be the direct moisture source. A sensitivity analysis for target height and adjusting threshold of specific humidity is also applied to the improved Lagrangian diagnostic. Overall, the findings were unaffected by different conditions, and the moisture was more likely to be locally sourced, as indicated by higher specific humidity thresholds.