Abstract: By using daily observatory precipitation data and NCAP/NCAR reanalysis data 1961—2010, this paper explores the influence of subtropical westerly jet on the onset of rainy season over Yunnan province and some new results are obtained. The results show that there exits astonishing variation of the subtropical westerly jet stream over East Asia during 10 days around the onset day of Yunnan rainy season: the jet stream over East Asia sways southward from 35°N to near 30°N rather than jumps northward. With a sequence of jumping northward of the subtropical westerly jet, swaying southward generally exists during the transitional season. When the subtropical jet sways southward and its upward branch of the secondary circulation around the entry zone is right over Yunnan province, the updraft would provide favorable dynamic disturbing condition for the rainy season onset.