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.