The three dimensional EOF(Empirical Orthogonal Functions) is used to study the South China Sea monsoon onset.The data is accepted based on the date of monsoon onset,from 30 days before onset to 30 days after onset.The upper and lower wind field are considered as a whole system.The first mode of the deviation wind field shows the rapid changes of the upper and lower circulation of the East Asian summer monsoon around the onset of South China Sea monsoon.It reflects the onset of South China Sea Monsoon and the abrupt change of the atmospheric general circulation;The second mode depicts the major area of mid-and high-latitude long wave action with 5—7 days period and the divergent part of deviation wind field;The third mode describes the major area of Asian summer monsoon with quasi-biweek low-frequency oscillation and the rotational part of deviation wind field.