In the late spring and early summer of 2005, Yunnan encountered weather with high temperature and drought, which is the worst in the recent 55 years. After the diagnostic analysis of convection and moisture in Southeast Asia, it was found that there is positive correlation between onset of summer monsoon over the Indochina Peninsula and establishment of rainy season of Yunnan province;If moisture transportation from Bay of Bengal is strong(weak) and convective activity around Sumatra island is active(non-active), establishment of rainy season of Yunnan is early(late).In 2005,the main causes of early summer drought of Yunnan are: The onset of summer monsoon over the Indochina Peninsula is later;Meridional moisture transportation from Bay of Bengal and convective activity around Sumatra island are weaker.