Abstract: Based on air-sea flux in-situ observation data in South China Sea (SCS), NCEP reanalysis dataset,GPCP, OLR and GMS-5 cloud image, this study synthetically analyzed establishment process of SCS southwest monsoon and related changes of rain belt in May and June 2002. Evidence shows that May 14 is the onset date of Xisha and north SCS, and May 15 for the whole SCS monsoon as a normal year. Remarkable changes of variables, such as wind direction, wind speed, cloud, precipitation, humidity, moisture flux, radiation and seasurface state, et al, were observed during the period of the monsoon onset. In the radiation components, variations in short-wave radiation and net radiation budget of ocean are especially significant. Abrupt variations in the lower and upper-level air structure are responsible to large-scale adjustment of the general circulation. The physical evolution of general circulation prior to and after the onset are as follow: firstly,cros-sequatorial flow in 80-90°E intensified, so did India-Burma low and air pressure gradient in Bay of Bengal, then the cyclone over East Asian developed and moved eastward, accompanied by the withdrawal of subtropical high. At some time equatorial westerly associated with the low pressure trough in Bay of Bengal enhanced and crossed Indochina Peninsula, leading to the buildup of strong southwesterly in north SCS, then finally SCS monsoon commenced in whole SCS. It is a cont inuous process from the intensificaion of southwesterly in Bay of Bengal to the onset of SCS monsoon. The southw ard movement of continental cold front plays an important role in triggering the onset of SCS monsoon. The earlier onset of the SCS monsoon in the northern part is due to the interaction of mid-latitude and low-latitude systems.At the time of SCS monsoon onset, it appeared single rain belt in the northern part of SCS, and then transited from single to double ones. The belts were affected by subtropical high and monsoon system, often following the movement of subtropical high. Onset periods of the monsoon (east ward withdrawal subtropical high) were associated with substantial rainfall over central and northern part of SCS. Break periods of the monsoon (westward shift of subtropical high) wererelated with minimal rainfall over northern part of SCS but substantial rainfall over the Bay of Bengal, Indian and Yangtze River basin. There were two different kinds of precipit ation patterns. Their variation period was about 30 days, which may reflect the intraseasonal oscillations of 30-40 days time scale.