Abstract: The coral reef is a special geotechnical type,its engineering characteristics is a new subject for the geotechnical engineer.There are fewer experiences in coral reef investigation,and no investigation code or norm exists at home.The borehole drilling is badly limited for the severe weather as well as the marine environment.Shallow marine seismic reflection is adopted in the investigation of a coral island in South China Sea,Combined with the borehole data,the seismic reflection figures out the geological stratigraphic sequence of coral clastic deposit and underlying reef limestone.The result achieved in depth mapping is quite satisfactory.It showes that the deposit interface in different reef phases is obvious,including of inner reef flat,atoll lagoon,seaward slope,offshore.Outer reef flat is somewhat unclear for the existence of hard modern beach rock.