Abstract: The time evolution of the general circulation over the South China Sea and surrounding areas during the period from April to June is studied using ECMWF data of 1980-1986. The first transition from the pre-onset (6-10 May) to the post-onset (11-15 May) 5-day subperiods of May is characterized by the distinct change of low-level (850hpa) winds from southeasterlies to southwesterlies along 15°N over the South China Sea and by the sudden movement of the center of South Asian High in the upper troposphere(200hpa) from 10-15°N to 15-20°N over Southeast Asia. Corresponding to the abrupt change in circulations, the gradients of the temperature and humidity intensify along 30°N over East Asia at 850hpa. The time sequence of the 850-200hpa layer thickness over this region shows that the layer-mean temperature increases abruptly at the same time. The corresponding sudden increase of the vertically integrated heat source < Q₁ > reveals that the heat source plays an evident role in the drastic changes. The time series of < Q₁ > over the northern part of the South China Sea shows that the abrupt increase of the areamean < Q₁ > is also found but it is 5-10 days laterthan the change of wind fields. The time series of Xisha SST shows its continuous increase to about 29.5℃ until 10 May when the abrupt changes occur.