Using the NECP data from 1983 to 1992, the spatial and the temporal va riation characteristics of main models of the summer monsoon onset process in th e South China Sea(SCS) have been studied by the method of EOF expansion. It has been found that the northward shift of solar radiation is the most important fac tor of the monsoon onset. There is very evident adjustment of wind field at 850 hPa associated with the Asian summer monsoon onset in typical monsoon area. Ampl ification of westerlies trough and ridge in Southern Hemisphere mid-high latitud e and augmentation of subtropical system meridional circulation in Northern Hemi sphere are significant role of the Asian summer monsoon onset. The western Pacif ic Ocean high pressure abated rapidly and retreated east about 5 days before the monsoon erupts, which caused the southwest monsoon onset.