Abstract: Seawater intrusion has become a common environmental problem in coastal areas due to rising sea levels and the overexploitation of groundwater. How to identify the characteristics of seawater intrusion of different causes and establish a scientific and effective evaluation system are the hotspots and difficulties of seawater intrusion research at present. In this paper, we focus on the muddy coastal zone area where the two types of invasion, ancient brackish water invasion, and modern seawater invasion, co-exist. We aim to identify the types of invasion and sources through an analysis of groundwater chemistry characteristics, this paper improves the original GALDIT method, and the improvements include substituting the degree of water enrichment (Q) in the area for the hydraulic conductivity coefficient (A) in the original model, and adjusting the original categorization of the other parameters, including the distance to the shore (D), and the impact of seawater intrusion (I), into better reflect and assess the actual situation in the study area. The results indicate that the original GALDIT model has limitations in assessing the coexistence area of ancient and modern seawater intrusion. The modified models, which include parameter replacements, modified parameter classifications, and the introduction of electrical conductivity (EC) as a key parameter, have effectively evaluated the groundwater vulnerability of the seawater intrusion area on the southern coast of Laizhou Bay, establishing a scientific foundation for developing treatment and management strategies for seawater intrusion along the south shore of Laizhou Bay.