Abstract: The Changchang Sag is located in the deepwater area of the Qiongdongnan Basin. It is linked with the northwestern sub-basin of the South China Sea through the Xisha Trough to the east side. The Changchang Sag trends in a nearly east-west direction,while the other basins in the deepwater area of the Qiongdongnan Basin are in NE—NEE direction. In order to better understand the crustal structure of Chang-chang Sag, deep reflection seismic profiles, VSP data and the latest marine gravity data were used for integrated geophysical simulation. The simulation results show that the crust of the north wing of the Changchang Sag is about 22~24 km in thickness, but 20~22 km of the south wing,decreasing greatly to the center of the Changchang Sag. The minimum thickness of the crystalline crust is 2.8 km. Mirror image relationship occurs between the Moho depth and the depositional basement, and the shallowest Moho is 13.8 km below the sea-level, corresponding to the deepest depositional basement. There is a high-velocity lower crust body in the east of the center of the Changchang Sag,which can be recognized on the deep reflection seismic profiles and layer velocity profiles. The crustal thickness decreases from east to west.