The Summer Somali Jet(SSMJ) onset dates during the period from 1948 to 2002 are decided by a defined index for SSMJ onset.Then the processes of SSMJ onset and the influence of anomalies of the winter land-sea surface temperature and the East Asia winter monsoon are analyzed in this paper.The results show that the onset of SSMJ is the earliest signal of the Asia summer monsoon;the onset of SSMJ is caused by the African-Indian Ocean System,including Mascarene High,Middle African Low and Arabian High;the developing Indian Low can force SSMJ eastward after crossing equator.The interannual anomaly of SSMJ onset dates reflects the interannual anomaly of prophase winter temperature difference between land and sea.When the winter monsoon is stronger(weaker),the onset of SSMJ is later(earlier) in the coming spring.