Coastal groundwater salinization is a global challenge, particularly in the North China Plain, but its hydrochemical evolution remains poorly constrained. To address this, we focused on the Cangzhou area as a typical coastal region. We employed an integrated approach combining hydrochemical analysis, environmental isotopes (18O, 2H, 14C), and SOM clustering to elucidate the spatiotemporal patterns and driving mechanisms of groundwater chemistry. The results indicated that the groundwater was classified into three groups: Group C1, brackish water with isotopically depleted SD and S18O signatures, was mainly distributed in the central and northwestern regions with intensive agricultural activity; Group C2, saline water with isotopically enriched SD and S18O signatures, was distributed in the central region; and Group C3, saltwater with the most enriched S18O and SD, was concentrated in the eastern coastal area. The hydrochemical type evolved from the Cl center dot HCO3-Na type in the west to the Cl center dot SO4-Na center dot Mg type and finally the Cl-Na type in the east, showing significant spatial zonation. Shallow groundwater (0-130 m) showed notable salinization and elevated ion concentrations, particularly in Groups C2 and C3. The S18O, SD and 14C results indicated that shallow groundwater in the central and eastern regions was significantly influenced by evaporation and paleo-seawater intrusion, whereas deep groundwater retained paleo-recharge characteristics under arid climate conditions. The chemical characteristics of groundwater in the central and western regions were mainly associated with water-rock interactions and agricultural activities. These findings provide a scientific basis for preventing groundwater quality deterioration and for sustainable water resource management.