Abstract: According to the definition criterion of this paper,the mean onset time of the South China Sea Summer Monsoon(SCSSM) from 1979 to 1994 is the Fourth pentad of MAY. The composite analyses of 16 years of OLR,wind field and θ_se field show that the onset of SCSSM is caused by tthe SCS Western Pacific System,In the lower troposphere,the weakening and retreating eastward of the Subtropical High in SCS region is the outstanding feature of SCS circulation system around the onset time of SCSSM.At the same time,the moving northwestward of the SCS High is the characteristics of SCS circulation systems in the upper troposphere.The evolution of potential instability and tropical deep convection develops earlier over the continent and island areas than over the sea,and the developing of potential instability is earlier than that of deep convection.The strengthening and the stretching eastward of 70-90° E equatorial westerly at the upper reaches of SCS region has direct effect on the onset of SCSSM.Meanwhile the enhancement of Somali cross equatorial flow and the retreating northward of Arabian High affect the onset of SCSSM indirectly through producing effect on the tropical westerly.