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In the transition zone between the Tibetan Plateau and the arid region of northwestern China, the spatiotemporal patterns and environmental controls of stable isotopes in precipitation remain unclear. A network of 19 sampling stations was established across the Qilian Mountains to observe stable isotopes in precipitation, and 1310 precipitation event-scale samples were collected. The local meteoric water line (LMWL) was obtained and expressed as δD = 7.99δ18O + 14.57 (R 2 = 0.96). The spatiotemporal patterns of the stable isotopes were mainly dominated by the co-influence of the water vapor sources and the local environment. The westerly circulation, monsoon circulation, and Arctic circulation accounted for 79%, 13%, and 8% of all precipitation events in the study region, respectively. The rainout process also caused oxygen isotope depletion for continuous precipitation events. When the temperature increased by 1 °C, δ18O increased by 0.47‰, but this increase varied with the temperature range. The effect of precipitation amount was apparent in summer and was caused by sub-cloud evaporation. In addition, δ18O decreased by 0.13‰ for every 100 m increase in altitude in the Qilian Mountains. Future research should focus on quantifying the co-influence of sub-cloud evaporation, local moisture recycling, and water vapor sources on stable isotopes in precipitation. Water vapor sources of all precipitation events in eastern, middle and western Qilian Mountains. Unlabelled Image • Transitional characteristics are evident for stable isotopes in precipitation. • Summer precipitation effects (-0.4 ‰/100m) caused by sub-cloud evaporation. • δ18O increased by 0.47‰ with a temperature increase of 1 °C. • δ18O decreased by 0.13‰ with an altitude rise of 100m. • The westerly circulation accounted for 79% of all precipitation events. [ABSTRACT FROM AUTHOR]