Saltwater intrusion is one of the most significant groundwater challenges in the southern Laizhou Bay. Previous studies have predominantly focused on regional scales, leaving the vertical saltwater intrusion pattern relatively underexplored. This knowledge gap hinders the effective prevention and control of saltwater intrusion. This study utilized hydrochemical and stable isotopic methods combined with hydrochemical facies evolution diagrams to investigate the groundwater evolution and the processes of saltwater intrusion in a typical profile and saline–fresh groundwater transition zones. The results showed that the groundwater types in the study area were complex and diverse, with fresh groundwater, saline groundwater, and brine. Stable isotope and hydrochemical analyses indicated that mixing and evaporation of seawater were the predominant processes governing the evolution and salinity of groundwater. In the south of the typical profile, carbonate dissolution played a significant role, and the silicate dissolution may represent the primary water–rock interaction in the saline–fresh groundwater transition zones. Groundwater samples from various locations within the study area exhibited different stages of hydrochemical facies evolution, and the majority of the typical profile samples were in the salinization phase during the mixing process. The saltwater intrusion in the saline–fresh groundwater transition zone primarily occurred between −20 and −30 m, exhibiting a wedge-shaped saltwater intrusion pattern. This study enhanced the understanding of vertical saltwater intrusion.