Abstract: Abstract: Integrated analysis and study of voluminous geophysical, geological and drilling data suggest that there are nine types of substrate on the sea bottom of the Daya Bay off the Guangdong coast. Except along the coast and around the islands where the grains of the substrate are coarser, the substrate in the shallow-sea area consists dominantly of clayed silt and silty clay, which has good engineering geological conditions. On the whole, the substrate in the bay is only slightly polluted at present, but because of the poor exchange capacity of waters in the bay, the substrate is easy to pollute and difficult to control; so efforts should be made to protect it. The potential geohazards in the bay include active factors, such as coastal erosion, sand waves and faults, and restricted geological conditions, such as irregular buried bedrocks, buried sedimentary ridges, sea-route gullies, abnormal accumulation areas and steep slopes.