Abstract: The Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT) platform, which was enhanced with the areal source–receptor attribution method, was used to simulate the Lagrangian trajectories of air parcels in eastern China during the summer monsoons from 1961 to 2010 at different vertical levels. At these levels, water vapor transport pathways and moisture contributions in each rainy season were quantitatively determined. During the pre-monsoon season in South China (SC), the dominant water vapor transport channel at the lower level (below 1500 m) was the West Pacific Ocean channel, with the proportion of trajectories reaching 52.3%, whereas at the middle level (1500−5000 m), the Indian Ocean channel was found to be dominant, with the proportion of trajectories exceeding 37%. The most important moisture source was the land area of East China and the West Pacific Ocean at the lower level; their contribution rate for each of the two sources exceeded 20%. After the onset of the South China Sea summer monsoon and during the entire monsoon season in SC, the Indian Ocean channel was the strongest moisture channel at the lower, middle, and upper levels (＞5000 m), and the proportion of trajectories at the middle level was 65.6%. Further, the moisture contribution from the Indian Ocean showed a significant increase, and it was the most important moisture source at the middle and upper levels. However, the most important moisture source at the lower level was East China and the South China Sea. During the Meiyu season, the West Pacific and Indian Ocean channels were the dominant channels at the lower level and at the middle and upper levels, respectively. Compared with the monsoon season in SC, the Indian Ocean channel was weaker at the middle and upper levels, whereas the mid-latitude westerly channel was stronger. During the rainy season in North China, the West Pacific and mid-latitude westerly channels were the dominant water vapor channels at the lower level and at the middle and upper levels, respectively. During the Meiyu and North China rainy season, the main source areas were eastern China and the West Pacific regions. In particularly, during the rainy season in North China, the water vapor from East China at the lower level reached 43.1%, which indicates that the local evaporation at the low level plays a crucial role during the rainy season precipitation in North China.