Abstract: The study of hydrogeochemical characteristics and genesis of groundwater is of great significance for rational utilization, protection and pollution control of coastal groundwater.Piper map, mineral phase equilibrium analysis, ion proportion coefficient and Gibbs analysis were applied to reveal the hydrogeochemical process, water-rock interaction, mineralization trend and origin of groundwater during spring tide and neap tide through the monitoring of Wells Z1 in Li'an Port, Z2 in Sanya Bay and Z3 in Banqiao in the south coast of Hainan.The results indicate that groundwater in Z1, Z2 and Z3 are saline water, fresh water and extremely salty water respectively, their hydrochemical types belong to Cl-Na, HCO₃·Cl-Na·Ca and Cl-Na respectively, and ions in the groundwater result from the dissolution of anhydrite, gypsum and halite.It is suggested that the groundwater sources of Z1 and Z3 are closely related to high-salinity seawater controlled by evaporation and concentration, while the hydrochemical environment of Z2 is mainly affected by weathering and dissolution of rocks with sodium silicate minerals.The above research results can provide basic data for the further monitoring of coastal groundwater environment and pollution control.