A Wind Direction Change Index(Iw) is designed to research the relations with the south China sea monsoon onset. The results show that by using the Yamamoto testing ,the key areas in the south China sea can be defined.There is wind field mutation of the key areas at 850hPa during the south China sea monsoon onset.The reverse of south China sea monsoon is a part of global wind field reverse. Iw space-time variation of key areas in the south China sea was analyzed.The wind at 300-30hPa begins to reverse in early May, which occurs about two weeks before the lower.It has the important meaning for the forecast of monsoon onset.