Abstract: This study primarily utilized precipitation observations from surface stations in Bayi District, Nyingchi City, together with GDAS reanalysis data, to quantitatively analyze the water vapor transport trajectories during June–August 2022 by means of a backward trajectory model, the potential source contribution function (WPSCF), and the concentration-weighted trajectory (WCWT) method. The water vapor sources and their contributions to summer precipitation were examined, and the relationship between water vapor transport and precipitation was further discussed in conjunction with the Plateau Monsoon Index (PMI). It should be noted that the WPSCF and WCWT methods have traditionally been applied to the source apportionment of atmospheric pollutants; in this study, however, they are innovatively extended to identify the water vapor sources of Bayi District, providing a new perspective for water vapor transport research. The results indicate that the outbreak of the South Asian monsoon enabled long-distance transport of moisture from the Bay of Bengal and northern Indian Ocean along the Yarlung Tsangpo River valley, making them the dominant water vapor sources of summer precipitation in Bayi District, with a combined contribution of 40%–45%. The sources of water vapor exhibited distinct monthly variations: in June, near-field valley recirculation was strongest (WPSCF = 0.70, WCWT = 15.9), together with short-range transport from southern Bomi; in July, during the monsoon peak, thickened background moisture layers diluted single-grid contributions, while the northern Bomi corridor became dominant (WPSCF = 0.10, WCWT = 3.09), resulting in more evenly dispersed sources; in August, as the monsoon weakened, long-distance transport through the mid–lower Bomi corridor reconcentrated moisture (WPSCF = 0.11, WCWT = 20.41), accompanied by a revival of valley recirculation efficiency. In addition to the main transport channels, local moisture recirculation (contributing 15%–20% in summer) and westerly moisture (primarily affecting non-rainy seasons) provided supplementary input to precipitation, functioning as a “low probability–high efficiency” trigger in complex orographic uplift regions. Further analysis, based on monthly precipitation records for 1981–2022, showed that the Pearson correlation between the Plateau Monsoon Index (PMI) and precipitation in Bayi District increased progressively from May to August. A significant negative correlation was observed in May (r = ?0.39, p < 0.05); the relationship approached zero in June, while significant positive correlations emerged in July (r = 0.38, p < 0.05) and August (r = 0.41, p < 0.01).In summary, the summer water vapor sources in Bayi District exhibited a seasonal evolution of “near-field in early summer—uniform in midsummer—long-range in late summer.” The interplay between the intensity of the South Asian monsoon and the valley channel topography jointly governed the regional water vapor flux and precipitation distribution. These findings provide a scientific basis for understanding moisture transport mechanisms and water resource management along the southeastern margin of the Tibetan Plateau.