Abstract: According to the basic characteristic of activities of South China Sea summer monsoon,an index of South China Sea summer monsoon intensity,Is,which is composed by both dynamic factor (southwest wind component) and thermodynamic factor (OLR) was studied in this paper. The results of Is of June,July,August from 1975~1999 have been calculated and the strong and weak monsoon years have been given. The characteristic of Is and the relationship between variations of Is and the date of the summer monsoon onset and the rainfall amount in Guangdong province and China have been analyzed. The results showed: the multi-year variations of the summer monsoon have ten years and 3-4 years periods in past 25 years. The earlier the summer monsoon setups,the stronger the summer monsoon is,and vise verse. In strong summer monsoon year,the rainfall amount in Guangdong is more than normal in the second stage of rain period and is normal or less than normal in the first stage of rain period. And the summer rainfall amount is more than normal in the northeast part of China,in the north part of middle China and south of Yangzi River and is less than normal in the west part of China. They are reverse in weak summer monsoon year.