Abstract: Stable isotopes in atmospheric water vapor, which can track moisture sources and water vapor transport, are extensively used as a crucial tracer of the present-day water cycle. To interpret water vapor stable isotopes in the mid-low latitude monsoon region, the “amount effect” is invoked. However, recent studies have demonstrated that nonlocal factors, such as moisture sources and water vapor transport, have a significant effect on stable isotopes. Thus, the Lagrangian Particle Dispersion Model and Satellite remote sensing deuterium isotope data (expressed by parts per thousand of their deviation, δD) in water vapor are used to investigate the primary factors affecting water vapor δD in the region with abundant Chinese stalagmite δ¹⁸O records. On the seasonal scale, water vapor δD is more depleted in late summer and early autumn and enriched in winter and spring. This characteristic is difficult to interpret in terms of “temperature effect” or “amount effect.” However, accumulated rainfall over water vapor transport paths is the dominant factor of water vapor δD, and there is a significant negative correlation between them. On an interannual scale, water vapor δD is enhanced in the summer of the El Niño year and depleted in the summer of La Niña year. The contribution of moisture sources to water vapor δD is small; however, the accumulated rainfall over water vapor transport paths increased substantially in the La Niña year compared with the El Niño year. This shows that in the La Niña year, tropical convection and depletion in water vapor transport paths are significant, resulting in depleted water vapor δD in the study area. Finally, on a seasonal to interannual scale, upstream convection, as measured by accumulated rainfall, is the primary driver of water vapor δD variations. In the study area, enhanced convection will deplete δD, whereas the weakened convection will enrich δD.