The June‐July Yangtze flooding in 1998 and 2020 drew incredible attention owing to the extreme precipitation events and devastating societal/economic damages. However, the quantitative estimation of the moisture transport mechanism is intensely discussed but still unresolved. Here we investigated two events from a unique perspective of Eulerian atmospheric water tracers that tries to explain the two events from model physics. The results showed that the moisture supplies from the Northwest Pacific decreased despite of different inducements, whereas the southwest summer monsoon (SWSM)‐related moisture supplies exhibited conspicuous enhancements in both events, suggesting that the SWSM‐related moisture supplies controlled the occurrence of Yangtze flooding. The physical processes of the two events were further compared. The 2020 flooding was more severe than the 1998 event, which was related to the stronger advective convergences and in‐stratus condensations of the SWSM‐related moisture. Plain Language Summary: In 1998 and 2020, the Yangtze River valley experienced extreme rainfalls during June–July. The associated atmospheric river was detected to be significantly enhanced by the moisture from the southwest summer monsoon‐related source regions (such as the Bay of Bengal, Arabian Sea, Tropical Indian Ocean, Indo‐China Peninsula, and India) in both events. On the contrary, the moisture supplies from the Northwest Pacific declined. These results suggested that the southwest summer monsoon‐related moisture supplies played vital roles in the formations of extreme rainfalls over the Yangtze River valley. Additionally, our study investigated the differences in the physical mechanisms of the two extreme rainfall events. The stronger advective transport and cloud processes forced more southwest summer monsoon‐related moisture to be converted into precipitation around the Yangtze River, ultimately leading to more severe flooding in 2020 than in 1998. Key Points: The southwest summer monsoon (SWSM)‐related moisture supplies regulate the emergence of the Yangtze flooding during June–JulyAlong with the flooding appearances, the moisture supplies from the Northwest Pacific to the Yangtze River valley decreaseStronger advective transport and in‐stratus condensations of the SWSM‐related moisture avail the larger flooding in 2020 than in 1998 [ABSTRACT FROM AUTHOR]