三江源区地处青藏高原腹地，是长江、黄河和澜沧江的发源地，具备重要的水源涵养功能，被誉为“中华水塔”。在气候变化背景下，该区域表现出气温上升和降水量增加等特征。充足的水汽供应是降水形成的必要前提，水汽的输送路径及其来源特征对三江源区降水机制、水资源调控和生态系统稳定具有深远影响。已有的相关研究多采用传统欧拉方法进行水汽输送分析，难以准确量化不同水汽源区的相对贡献；而采用拉格朗日轨迹模型的研究则多聚焦于极端降水事件，缺乏长时间的系统分析。本研究使用CFSR再分析数据驱动拉格朗日模式FLEXPART，对1980-2017年间全球500万个气块进行了长时间的前向模拟。结合Water Sip水汽源诊断方法，聚焦三江源区的雨季（5-9月）降水，系统地识别和分析了水汽的主要输送路径、各水汽源区对降水的贡献，并进一步探讨水汽源地时空变化特征及与主要大尺度环流系统之间的关系。结果发现，影响三江源区雨季降水的主要水汽通道可以归纳为两支：一支为随中纬度西风带由西部或北部进入，特点为数量众多、海拔较高但水汽含量相对较低；另一支为随亚洲季风由南部或东部进入，虽然数量较少，但途径海洋区域，海拔较低、水汽含量丰富。使用Water Sip方法诊断水汽源的结果表明，三江源区雨季水汽源呈现出以青藏高原为中心并向外扩展的分布特征，其中向南亚季风区域扩展最为突出。青藏高原内部水汽循环对三江源雨季降水发挥了主导作用，贡献了66. 3%的水汽；其次为南亚季风区，贡献率了18. 87%的水汽；西风区和东亚季风区贡献相对较少，分别贡献了 7. 00% 和 3. 08% 的水汽。从时空变化特征来看，1980-2017年间三江源区雨季水汽源以增加趋势为主，其中青藏高原内部及喜马拉雅南麓区域最为显著，超过4 mm·（10a）-1。这与南亚季风和高原季风的持续增强有关，提升了南亚季风区、青藏高原内部和周边区域的水汽输送能力。青藏高原西部至中亚地区的水汽贡献量呈现弱的下降趋势，这与中亚西风急流位置南移和强度减弱密切相关。东亚季风年际变化不显著，其对三江源区雨季水汽输送的调控作用相对较弱。

The Three-River Source region（TRSR），located on the Qinghai-Tibet Plateau（QTP），are the source regions of Yangtze，Yellow and Lancang River，plays a critical role in water conservation and is renowned as the “Water Tower of China”. Under the background of global climate change，this region exhibits characteristics of rising temperatures and increased precipitation. Sufficient moisture supply is a prerequisite for precipitation，and the transport pathways and the characteristics of moisture sources have profound impacts on precipitation mechanisms，water resource regulation，and ecosystem stability in the TRSR. Current research predominantly employs traditional Eulerian methods for moisture transport analysis，which makes it difficult to accurately quantify the relative contributions of different source regions. Studies using Lagrangian trajectory models often focus on extreme precipitation，lacking long-term analysis. This study used CFSR reanalysis data to drive the Lagrangian model FLEXPART，conducting long-term forward simulations of 5 million air parcels from 1980 to 2017. Combined with the Water Sip moisture source diagnostic method，the research focused on the rainy season（May-Sep‐tember）in the TRSR，systematically identifying and analyzing the main moisture transport pathways，contribution proportions from various source regions，and further exploring their spatiotemporal variation characteristics and relationships with major large-scale circulation systems. Results indicate that the main moisture transport channels affecting precipitation in the TRSR during rainy season can be summarized as two types：one enters from the west or north along the mid-latitude Westerly，characterized by numerous air parcels at higher altitudes but with relatively lower moisture content；the other enters from the south or east along the Asian monsoon，al‐ though the quantity is relatively small，the routes pass through marine areas with low altitudes and rich moisture content. Moisture sources diagnostics using the Water Sip method reveal that the rainy season moisture sources for the TRSR exhibit a distribution pattern radiating outward from the QTP，with the most prominent expansion towards the South Asian monsoon region. The internal moisture cycle of the QTP plays a dominant role in precipitation in the TRSR during the rainy season，contributing 66. 3% of the moisture. The South Asian monsoon region ranks second，contributing 18. 87% of moisture，while the Westerly region and East Asian monsoon region contribute relatively less，at 7. 00% and 3. 08% respectively. From a spatiotemporal perspective，moisture sources during the rainy season in the TRSR showed an overall increasing trend from 1980 to 2017，with the most significant increases from the QTP and the southern of the Himalayas，exceeding 4 mm·（10a）-1. This trend is closely related to the strengthening of the South Asian monsoon and plateau monsoon，which enhanced moisture trans‐ port capacity from the South Asian monsoon region，the interior and surrounding areas of the QTP. The moisture contribution from the western QTP to Central Asia showed a declining trend，mainly affected by the southward shift and weakening intensity of the Central Asian Westerly. Meanwhile，the trend in the intensity and position of the East Asian monsoon were not significant，resulting in a relatively weak effect on moisture transport to the TRSR during the rainy season.