Abstract: Based on the survey data from the Central Yinggehai zone, some undulating hard seafloor is observed. The Side Scan Sonar and multi-beam data suggest that the reflection of the hard seafloor is distinct or slightly stronger than that in surrounding areas. The distribution of the hard seafloor changes from place to place, and mainly occurs with tongue-shaped topography. Stratigraphic profiles show that the hard seafloor is characterized by very thin layer A or without the layer A. Sampling results show that the layer A is very soft brown gray clay and the underlying layer B is hard to very hard brownish yellow silty clay, suggesting an oxidational depositional environment. Further study suggests that the hard seafloor consists of the deposits which deposited in the area out of paleo channels and lakes and distributed near the border. Evidence shows that the hard seafloor was formed during the regressive period. The characteristics and engineering significance of the hard seabed is also analyzed and discussed in this paper. The hard ground is important for constructions of submarine pipelines and various coastal construction programs.