Abstract: The regional structural geology，igneous rocks and spatial relationships among the springs, as well as the springs’ hydrochemistry and isotope datum were analyzed. The characteristics of thermal springs in Hainan Island could well be concluded by this study, and formation of thermal underground water can be detected. The thermal springs distribute over northwest and southeast areas which are located at lower altitude. The type of water is mainly HCO^-₃ dominant, and slightly alkaline, high fluorine and silica. However, those from Quaternary beachfront region are the type of NaCl dominant because of seawater influence. Data on hydrogen, oxygen, helium isotopes and gas chemical composition showed that the underground thermal water was originated from local precipitation, then this precipitation was heated by the surrounding rocks when it cycled in earth’s upper crust and warmed to hot underground water finally. Geothermal source belongs to a low-medium temperature convectional geothermal system, and equilibrium temperature of the geothermal reserve in the range of 59.5-161.2℃ with the average temperature of 128.5℃. The heat energy is not influenced by magmatic activities but related to the higher regional geothermal flow in deeper region. The out-crops of thermal underground water are controlled by the regional NE, EW trending faults.

Key words: thermal spring, thermal underground water, fault, hydrochemistry, isotope, Hainan Island