Abstract: In this paper, the variation of the atmospheric dynamic,thermodynamic structures and the moisture,energetic budget over the southern China before and after the onset of summer monsoons was investigated. The results show that the establishment of monsoons over the South China Sea is an adjustment process of the general circulation, which happens mainly in the middle and upper troposphere. The main features are: the southern Asian high is shifted northwestward into the Indo-China Peninsula in the upper troposphere, whereas the subtropical high over the South China Sea stretches westward for a few days and then withdraws eastward in the middle-lower layer. The eastward retreat initializes the SW monsoon flow all over the South China Sea. As a result of this change, strong rising motion takes the place of sinking motion. The amount and direction of vapour transport change before and after the onset, however, significant increase in vapour convergence and heavy rainfall occurs only after the onset. All these features are obviousely different from those of the Indian monsoon onset process, which is characterized by the monsoon onset vortex, and the simultaneous advance and feedback mechanism of the SW monsoon and its rainfall. The change of energy budget is also distinctive. Before the onset, a large amount of kinetic energy is converted into the total potential energy. Then part of it is exported outside the region and the rest is consumed by the diabatic cooling in the upper troposphere. After the onset, the energy cycle is almost reversed. The diabatic heating in the troposphere generates a large amount of total potential energy, and the convergence term of potential energy flux also makes a positive contribution. This increase of potential energy is converted into the kinetic energy to maintain and speed up the summer monsoon over the South China Sea. It is also shown that the change of wet hydrostatic energy after the onset is mainly determined by the change of latent heat. In other words,the latent heat released by the convective process is the most important factor to the maintenance of the Indian monsoon and the East Asian monsoon.