Abstract: The gravity gradient anomaly reflects the change of gravity anomaly,and it has a higher resolution than gravity anomaly.The plot of gravity gradient space parameters can give diagnostic information of dip angles and planes.A geometric model of subsurface structure may be constructed by integrating a gravity gradient section with the gravity gradient space parameter plot,and furthermore some complex structures may be interpreted.The authors used gravity gradient anomalies to interpret the Southwest sea basin of the South China Sea and obtained the domal structure plane with the NE-trending spreading axis of the sea basin as the symmetric axis.This structure plane forms a density interface 6-15 km below the sea basin,which might be caused by mantle uplift during NW-SE-trending sea floor spreading of the Southwest sea basin.