Laboratory experiments in sediment quality at the hollow places are carried out in the presenting paper. They are designed to have the correct information of sediment in hollow area and to clarify the influence of the sediment on water quality in coastal areas. The sediment qualities in the hollows are much worse than those outside of the hollows, so that the rate of oxygen consumption by the bottom sediment in the hollows is as 2-10 times as that outside of the hollows.The formation rate of inorganic phosphorus and nitrogen in the sea mud and the diffusion rate of inorganic phosphorus and nitrogen to seawater in the hollows are much higher than those outside of hollows. These rates are different between sediments in two hollow places because of sulfide form.