Abstract: The data of 2014 from monitoring wells in South Coast of Hainan Island was studied during the times of neap tide and spring tide. Variation in chemical elements of the groundwater is adopted for revealing its relation with seawater intrusion. Our results show that chemical element concentration varies with tidal cycles, location of wells and surrounding environment. In the wells of QZ2 and QZ5, the contents of K⁺, Na⁺, Ca²⁺ are quite close, but Mg²⁺ is obviously different. In the well QZ2, it is 18.82% higher than that in well QZ5, suggesting that the well QZ2 is more severely affected by seawater. Na⁺ and Cl^- are the dominant ions in both the wells of QZ2 and QZ5. The average ratio of Na⁺ is 83%~85%, Cl^- is 87%~88%, showing effects of seawater. In the well QZ3, HCO^3- is the dominant, the content is 62%~63%, and Ca²⁺ also occupies a large proportion, showing the predominance of fresh water. The SAR of QZ2 and QZ5 wells are 57.95~ 63.24 and 57.20~63.13 respectively. However, it is 2.17~4.42 in well QZ3, obviously lower than those in QZ2 and QZ5. As the conclusion, wells QZ2 and QZ5 have higher risk of salinization.