We collected and processed the site velocities of South China Sea(SCS) and its surrounding areas and obtained unified velocity field in ITRF2005. By analyzing the spatial variation of the velocity field and strain field, and relative motions between blocks it is discovered that the SCS extends still up to now. The western part of SCS is opening gradually NWSE from its northern margin to south, but the eastern part of SCS is opening gradually from its central part to north and to south. On the basis of computed results of strain field, the average extension rate in about
SN is 3.1 mm/a in the CSB and extension rate in SN is 9.3 mm/a in eastern part of SCS 117°E between 11°N and 22°N which is smaller than the average extension rate of 13.0 mm/a along NNWSSE since 65 Ma. The expansive motion of the SCS is resulted from the common effects between Indian plate, Pacific plate, Philippine Sea plate and Eurasian plates, and the upwelling of the mantle matters of SCS area.