By using the NCEP/NCAR reanalysis data averaged over 40 years from 1958 to 1997,the climatic characteristics of the tropical circulation in the two Hemispheres are analyzed.The relations between the onset of the South China Sea(SCS)monsoon and the large-scale circulation are discussed.It is indicated that the updrafts of the Arabian Sea meridional cell tube and the south zonal cell tube arrive in the SCS at the same time in May,which brings strong convective precipitation.Meanwhile,the westerly of the meridional cell tube and the southerly of the south zonal cell tube combine together to form the southwesterly at the low levels,resulting in the onset of the SCS monsoon.The influences of the Tibetan Plateau and the land sea distribution at different longitudes cause the heating differences of solar radiation,which is the direct cause of the summer monsoon onset,as well as,the cause of the earlier onset in the SCS than in the Indian region.