Abstract: Based on the marine ship obser vation data, records of the meteorological stations and HRC data by satellite remote sensing with long terms, the circulation patterns and variability in elements during the onset and the established periods of the South China Sea south west(SW) monsoon have studied. The averaged date of the SW monsoon onset over the SCS occurs in the middle of May climatically. The correspo nding date for the northern part is little earlier(May 12) and those for the southern parts are little later, May 20, respectively. The interannual range of the onset dates is about one month. Following the onset of the SW monsoon, the cloud amornt and the precipitation increase while the convection activities enhance over the SCS. But there is a strong spatial variation within the domain. After onset of the SW monsoon the strong convective area moves north wards, while the SCS rain band moves to the center and north. The sea surface temperature increases rapidly before the onset and the leading time is one month. The increment of SST supplies heat and vapor for the onset. In April, the surface heat fluxes display obvious changes, e. g., latent heat exchange and evaporation enhancement. It is one of the reasons why the SW monsoon bursts firstly over the SCS.