Study is undertaken of South-China Sea (SCS) monsoon onset and Asian monsoon establishment features alongside possible mechanisms in the context of 1980～1986 ECMWF gridded data and 1980～1992 Japanese GMS derived TBB data. Results show that the precursors of Asian summer monsoon establishment appear first in the Indo China peninsula to the SCS, followed by west and east spreading; the Indo China is the division between east Asian and Indian monsoon, with the establishment and genesis mechanisms quite different on both sides of the peninsula; Asian monsoon establishment from east to west is closely related to the turning of meridional temperature gradients, their maxima travelling westward and the west spreading of positive temperature difference from the Indo China when winter goes to summer.