Abstract: In this paper, YIN-MIAN trough during summer in the years of 1974-1983 are analysed. It is found that the YIN-MIAN trough is a branch of Indian monsoon trough and its activity is restricted by the tropical weather systems over the South China sea and adjacent area.The favorable condition for the YIN-MIAN trough genesis is at the late regime of monsoon surges which originate from the cross-equatorial current from the Southern Hemisphere and prevail over the Bay of Bengal and Indo-China Peninsula. It is shown that there is close relationship between the activity of the YIN-MIAN trough and the haevy rainfall over the South China in the early rainy season.