Based on ECMWF daily objective wind analysis study is conducted of the onset and structure of summer monsoon in the South-China Sea. Results show that the monsoon at low levels passes through a seasonal cycle of four phases: 1) the northeasterly prevailing in winter; 2) the southeasterly blowing as the transition from late January to mid April; 3) the vigorous SW wind in summer; and 4) again, the transitional southeasterly spanning early October to early November. In particular, the southeasterly establishment over the sea in late Januaryearly February and spreading northeast-and northwestward serve as a triggering factor of summer SW monsoon dominating all over Asia. The monsoon has structure of thermally strong NE-SE-SW winds in order from top to bottom in the troposphere. In addition, the lowlevel monsoon flowfield clearly shows that the study monsoon consists largely of SW flow on the west side of Western Pacific subtropical high, 105～120 °E cross-equatorial veering flow from the Southern Hemisphere and SW flow of Indian Ocean origin, indicating the unique role of the research monsoon in bridging the counterparts in South Asia and subtropical East Asia.