Abstract: By using wavelet transform method to the 10-day precipitation rate of the NCEP reanalyzes data for the 17-year period from 1979 to 1995, the multi-hierarchy structures in time and space of precipitation over the South China Sea (SCS) are studied. The onset and development of the SCS monsoon are analyzed, and the onset mechanism of the SCS monsoon is discussed. The conclusions are as follows: (1) The onset of the SCS monsoon occurs around mid-May. The onset process is that the small range (32 longitudes) precipitation adjusts to the large range (64 longitudes) precipitation. Once the large precipitation takes place and arrives at the SCS, it is the onset of the SCS monsoon. After the adjustment, onsets of the Indian monsoon and the Eastasian monsoon begin in succession. (2) To the north of 10°N,the monsoon onset first occurs in the SCS, then propagates westward gradually to India. (3) The SCS region can be divided into three subregions:the northern (20～22°N), the middle (10～20°N) and the southern (to the south of 10°N). (4) In the area of the SCS monsoon,the annual cycle is very evident, and the oscillations of 30～60d and 20～30d are also evident. Besides these, there is a period of around 3-month. (5) The onset of the SCS monsoon is the result of the interaction between the low-frequency oscillations of 20～30d and 30～60d.