Based on the NCEP reanalyzed data,OLR and TRMM,the characteristic of the activities of summer southwest monsoon which influenced China in 2003 are analyzed. The establishment of South China Sea summer monsoon(SCSSM) ,the movement of summer monsoon over China,the intensity of summer monsoon and the intraseasonal oscillation of SCSSM are discussed. The results show that the establishment of SCSSM was later than normal and the intensity was weaker than normal. The intra-seasonal oscillation of SCSSM played an important role in severe flood in the Huaihe region.