Southwest China (SWC) is located in the transition zone affected by the East Asia summer monsoons, India monsoon, and the Tibetan Plateau thermal activity. SWC is also an important hub for water vapor transport in the Asia–Pacific (AP) region. Here we study the dynamical and spatiotemporal characteristics of extreme rainfall events in AP by a synchronization network approach, providing insights for understanding of the relevant synchronization mechanisms of extreme rainfall between the SWC and AP. It is found that the probability of significant synchronization rainfall events between SWC and AP as a function of distance changes from a power-law decay behavior to a teleconnection one at around 2000 km. Specifically, the SWC—the middle and lower reaches of the Yangtze River—Japan band (SYJ) is an important teleconnection pattern of extreme rainfall in AP region. This SYJ teleconnection pattern is associated with the water vapor flux convergence along the main water vapor transport route from the low latitude ocean. Moreover, we find that the inter-annual variation of the SYJ teleconnections index can reflect the evolution of synchronization rainfall features from SWC to Japan, which could be useful for operational climate monitoring and prediction. It is also found that the SYJ index is strongly correlated with the sea surface temperature anomalies (SSTA) over the North Indian Ocean and the Western Pacific. The warm Indian Ocean and Western Pacific SST can motivate the transfer of water vapor from SWC to Japan, yielding the synchronization of extreme rainfall along the SYJ band.