Abstract: Seawater invasion has been a common environmental problem in the coastal areas due to the sea level rising and the overextraction of groundwater. How to identify the characteristics of seawater invasion with different causes and establishing a scientific and effective evaluation system have become the hotspots and difficulties in the research of seawater invasion at the present. In this study the muddy coastal zone along the southern shore of the Laizhou Bay, where both ancient brackish water invasion and modern seawater invasion co-exist, is taken as the study area. The types and sources of the invasions are identified by means of chemical analysis of groundwater in the study area. The original GALDIT model used in the study is imporoved and the mordified contents mainly include replacing the hydraulic conductivity coefficient (A) in the original model with the water storage capacity (Q) within the study area. In order to more accurately reflect and evaluate the actual conditions in the study area, the original categorization for other parameters (e.g. the distance away from the coast (D) and the impact of seawater invasion (I)) is also adjusted. The results indicate that the original GALDIT model has some limitations in assessing the coexistence area of ancient salwater and modern seawater invasions, and the modified model, due to replacement of parameters, adjustment of parameter classifications according to historical invasion records and introduction of electrical conductivity (EC) as a key parameter, can be used for effectively evaluating the vulnerability of groundwater in the seawater invasion area along the southern coast of the Laizhou Bay, thus providing scientific basis for developing the measures of control and management of seawater invasion in the muddy coastal zone along the southern Laizhou Bay.