Daily surface observation precipitation datasets from 1961 to 2010 and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis datasets were used to perform rotated empirical orthogonal function (REOF) and cluster analyses in determining the climatic characteristics of the early summer rainfall and relevant atmospheric circulation in the southern part of Jiangnan, which is the southern region of the Yangtze River. The results reveal a concentrated rainy period in the region, which is the early summer rainy season independent of the pre-flood season in South China and the Meiyu in the Yangtze-Huaihe River basin. This rainy season has an average duration of 20 days and generally begins on June 11, approximately 8 days before the start of the Meiyu. In addition, movement westward and eastward of the West Pacific subtropical high (WPSH) provide important circulation backgrounds for this rainy season. That occurring after the WPSH extended westward abruptly and rapidly in the pentad of June 5-10. The west ridge points of the WPSH at 850 hPa remained steady near 133°E, which is its westernmost point during the rainy season, then the early summer rainy season ended as the WPSH retreated eastward and shifted northward at the end of June. Moreover, low-level jet (LLJ) patterns are significant dynamic factors during the early summer rainy season. The LLJ at 850 hPa from the Bay of Bengal and Indian Ocean connected with the wind current at the northwest side of the WPSH and coupled with the upper-level westerly jet at the region of concentrated precipitation located to the left of the LLJ and to the right of the upper-level jet inflow boundary precisely in the southern part of Jiangnan.