Abstract: The seawater-recharged geothermal system in the coastal areas of Xiamen City has abundant recharge source, however, the geothermal water is always saline and low-temperature, and the exploitation of geothermal water in these areas may even incur seawater intrusion. Therefore, to ascertain the hydro-chemical characteristics and genetic mechanisms of the geothermal resources is significant for the rational exploitation and protection of these resources. In this study, the chemical and isotopic compositions of the geothermal water, cold groundwater, and surface water samples collected from a geothermal field were analyzed. The results showed that in the mountainous and the piedmont zones of Xiamen City, geothermal water is mainly recharged by the precipitation and is dominated by HCO₃·SO₄-Ca·Na water with low TDS content. By contrast, the geothermal water in the coastal areas is mainly recharged by seawater mixed to different degrees and is dominated by hydro-chemical type of Cl-Na with high TDS content. Based on the characteristics of the Cl^-mixing model of 13 geothermal fields in Xiamen, 10 geothermal fields are recharged by seawater, among which the maximum seawater mixing ratio in geothermal water is 73.20％ in Pubian area. The geothermal water infiltrates in the low mountainous areas and is transported through NW-trending faults to regional deep-rooted geothermal-controlling NE faults, where the geothermal water receives the heat conducted from deeper parts. The temperature of the deep geothermal reservoirs was estimated from 185 ℃ to 225 ℃. While rising along faults, geothermal water was recharged by seawater and cold groundwater, creating shallow geothermal reservoirs with temperatures from 80 ℃ to 139 ℃. As indicated by the comprehensive analysis, seawater is an important recharge resource for geothermal water in the study area and significantly influenced the chemical components of the geothermal water. In addition, the geothermal water could be mixed with cold groundwater or seawater twice or more times, reducing the temperature of the geothermal reservoirs.