As an important geochemical element, cation Ca2+ has important significance in groundwater cycle and hydro-litho interaction especially in seashore zones. In order to understand Ca2+ behavior and variation in above zone, shallow groundwater sampling during tidal cycle and laboratory analysis was carried out around seashore of Hainan Island. The results show that the contents of calcium ion are different and fluctuates due to the influence of groundwater exchange and seawater intrusion, and the fluctuation of calcium ion occurs, which is non - conservative and active. The chemical species and minerals of calcium ion were drawn in the simulation, and results showed that CaSO4、CaHCO+3、CaCO3、CaOH+、CaHSO+4 most occurred free. Charged particle activity is most affected by free state, and calcium-containing minerals are mainly composed of sulfate and carbonate. Seawater intrusion also caused lower activity of free Ca2+ and more dynamic in resolution-precipitation equilibrium. According to free Ca2+ and other free cations resulted from PHREEQC modeling, their linear statistics also indicated that Ca-Na exchange was dominant and of anti-direction during sampling cycles. Generally, groundwater in northeastern parts of Hainan Island were of high relative cation exchange capacity (CEC) and southern and northwestern of high absolute CEC. It is concluded that the Ca2+'s behavior and thus their indication should be paid further attention in southeastern China land-sea transition immediate zone because of seawater intrusion.