Abstract: To reveal the evolution of groundwater chemical composition and the complex hydrogeochemical interactions due to the seawater intrusion in Xingcheng coastal plain, Liaoning, NE China, 44 groundwater samples and 1 seawater sample were collected from the Caozhuang Town section at Xingcheng River to Shahouzuo Town section at Yantai River. The chemical composition, and hydrogen and oxygen isotopes of the samples were determined. The hydrogeochemical characteristics and formation processes in typical seawater intrusion areas and their variations were analyzed. Results show that, the chemical field of groundwater in the area exhibited obvious horizontal zonation, and the water quality changed from freshwater to brackish water and to saline water successively from inland to coastal areas. The chemical types of groundwater were mainly HCO₃-Ca, HCO₃·Cl-Ca, and SO₄-Ca, while those of brackish water and salt water were mainly Cl-Na or Cl-Ca. Groundwater in the area was recharged by mainly atmospheric precipitation, with groundwater in the inland freshwater mainly controlled by dissolution and filtration processes, while brackish water was influenced by evaporation and mixing processes, leading to oxygen drift; . (3)The mixing ratio of seawater represented by Cl component with freshwater in brackish water ranges from 0.45% to 3.46%, while the mixing ratio of saline water was about 15.85%. The formation of brackish water and saline water was due to the combined effects of mixing, evaporation concentration, and cation exchange adsorption.