Fluorine is micro-nutrient which is essential for human health, and the excessive intake or deficiency causes health problems. High-fluorine groundwater in coastal plains has been widely documented in China and other countries, including the northern Shandong province of China. The groundwater fluorine levels, its distribution, epidemiology and health-related impacts in the coastal plains of the northern Shandong province were widely investigated, but its enrichment mechanisms and influencing factors are still unknown. Seawater intrusion, a common geological process, greatly changes groundwater geo-chemical properties, especially resulting in high pH, Na⁺, TDS, Cl^- and low Ca²⁺. Additionally, the groundwater fluorine is elevated under the conditions of high pH, Na⁺ and low Ca²⁺. But the process of seawater intrusion has not been noticed when the enrichment mechanism is discussed. The groundwater samples in typical fluorosis area of the northern Shandong province were gained. The high-fluorine groundwater is characterized by seawater intrusion, with high electrical conductivity (EC), total dissolved solids (TDS), hardness, Cl^-, Br^-, HCO₃^-, Li⁺, Na⁺. The fluorine levels in groundwater was positive correlated with Ec, TDS, Cl^-, Br^-, Li⁺ and significantly with Na⁺, Na⁺/Ca²⁺ and sodium adsorption ratio (SAR). The lab simulation experiments of fluorine leaching ability was performed. It is confirmed that fluorine leaching-ability of sediments in the aquifers increases with the higher ratios of seawater or brine water, higher levels of NaCl and NaHCO₃, and the lower levels of CaCl₂. Therefore, seawater intrusion should be the important dynamic of fluorosis, which is new insight into groundwater fluorine enrichment in coastal plains.