Based on the national monitoring standard of rainy season in Southwest China (SWC) issued by the China Meteorological Administration (CMA), and daily precipitation amount at 2513 meteorological stations in China and the NCEP/NCAR reanalysis project, climate features of the beginning date of the rainy season and its relationship with general circulations are analyzed in this paper. Results indicate that the mean beginning date is the fourth pentad in May. During the research period (1981-2014), the beginning date showed an advancing trend since 2000. Compared to the cross-equatorial flows (CEFs) over the South China Sea, the western Pacific and the Papua New Guinea, statistical results reveal a more significant relationship between the beginning date and the CEFs over Somalia (SCEF) and over the Bay of Bengal, especially the former. Stronger (weaker) SCEF is related to earlier (later) rainy season and more (less) precipitation in SWC. Both lagged correlation and composition analysis reveal a 10-day leading influence from the meridional wind along the SCEF channel. Since ten days before the beginning of the rainy season, the SCEF starts to strengthen continuously. Affected by the SCEF, both the westerly winds over the tropical Indian Ocean and the southwest monsoon transport enhance and bring more moisture to Southwest China, triggering the onset of rainy season in the region.