Abstract: Based on sea-level and upper-air data for the South China Sea and its adjacent areas, the distribution of monsoon indexes and their monthly and seasonal changes are oomputed, the source areas and trends of advancement or retreat of the monsoon analysed from the point of low-level flow fields, anc the advancing route and intensity of the monsoon disoussed. The result indicates that the summer monsoon for the area first appears in the Gulf of Thailand and then spreads to the northeast; the earliest surges reaching the South China Sea originate from the southeasterly winds to the south of the Subtropical High and from a cross-equatorial passage ast 90°E, 105°E later ; only when the summer monsoon is established in the South China Sea would the Indian Monsoon merge into it. The intensity, thickness and affecting area of the winter monsoon is also discussed in detail.