Abstract: The mechanism of saltwater intrusion through the salt-fresh water interface in a muddy coast is a hotspot of current coastal hydrogeological studies. The frequent shifting of palaeo-environments in the muddy coast caused by transgressions is the background of modern saltwater intrusion. Human activities, such as coastal land reclamation, have accelerated the process of modern saltwater invasion. The excessive exploitation of groundwater or brine water will certainly destroy the existing hydraulic balance. And as the result, saltwater will invade through the salt-fresh water interface, driven by the difference of the lower freshwater head and the higher saltwater head. The research progress of seawater intrusion through saltwater-freshwater interface in a clayey silt coast was summarized in this paper. Based on the review, we suggest that the study on the mechanism of saltwater intrusion through salt-fresh water interface be strengthened, and suitable measures be taken to protect and/or restore the hydrogeological environments in muddy coastal areas.