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Under the wintertime background of westerly flows, heavy rainfall often occurs first over southwestern China and then spreads eastward downstream to southeastern China, causing serious impact on a large scope of southern China. By using the synoptic diagnosis and dynamic analysis methods, we examined a typical eastward-moving heavy rainfall event that occurred over southwestern China on January 8–9, 2015 and then gradually spread eastward to southeastern China on January 10–13. The results show the important role of the Rossby wave train propagating along the subtropical westerly jet in “pushing” the eastward movement of heavy rainfall over southern China. The remote Rossby wave train influenced the variation of heavy rainfall by modifying the local circulation. The Rossby wave train can change the morphology of the East Asian subtropical jet via wave—jet interactions and modify the related secondary upward circulation in the entrance area of the jet, providing favorable dynamic conditions for the occurrence of heavy rainfall. The Rossby wave train can also influence the low-level south branch trough by modifying the associated baroclinic energy conversion process under a background of deep layer subsidence, which favors the transport of plentiful water vapor to the region of heavy rainfall over southern China. The eastward-propagating Rossby wave train pushed the upper level East Asian subtropical jet and the lower level south branch trough as a whole while simultaneously moving east, which caused the region of heavy rainfall to move from southwestern to southeastern China. These results will help to provide a scientific basis for forecasting winter rainfall over southern China.