An estuary is a transition zone which connets river flow with seawater. After the river-borne sediment is transported into estuary, the sediment is to be gradually deposited because of the flow diffusion, decreasing the sediment-carrying capacity. Shipping is hindered by the deposited sediment which is ususally aggragated somewhere on channel bed to form an entrance bar. The cause on the formation of the entrance bar had been contributed mainly to the phenomena of the estuarine circulation, sediment deposition from flocculated suspensions, and the turbidity maximum. However, these phenomena result from seawater intrusion. This paper reviews the understanding and the research advances on the effects of saltwater intrusion in estuary at home and abroad as well as the progress in research at the present time.