Based on the monthly mean sea level pressure data during the years of 1951-1994,the East Asia summer monsoon intensity index has been suggested. The statistical results show that the variation of summer rain belt pattern relates closely to the strength of summer monsoon in the East Asia. It was found that when the strong monsoon years,the positive anomaly of precipitation would appear in the north China (Pattern Ⅰ rain belt),when the week monsoon years,the summer rain belt would appear in the south China (Pattern Ⅲ rain belt) ；when the normal monsoon years,the summer rain belt would appear in central China (Pattern Ⅱ rain belt). The onset time of summer monsoon in the East Asia is one of the factors affecting the strength of summer monsoon. The stronger summer monsoon is predominant when the summer monsoon established early in East Asia ion the contrary,the weaker summer monsoon is predominant. These results would be useful for the long range forecasting of rainfall anomaly pattern.