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The hydrogeochemical characteristics and isotopic analyses of geothermal water are effective in the genetic study of a geothermal system. This study systematically investigated the geochemical and isotopic compositions of the geothermal water in the Qutan and Reshuizhou geothermal fields. The geothermal water in these two fields is of HCO3 (·F)-Ca·Na type and has undergone protracted water runoff and deep thermal cycle. Based on the mineral-water solubility equilibrium, the reservoir in the Qutan geothermal field has a temperature of 60–74°C and a depth range of 770–1003 m. The geothermal water in the Reshuizhou geothermal field has been mixed with cold water, which accounts for 58% in weight. The reservoir in the Reshuizhou geothermal field has a temperature range of 131-150°C and a depth range of 1953–2287 m. The geothermal water in the Qutan and Reshuizhou geothermal fields have 14C ages of 8251–7961 BC and 3514–2254 BC, respectively, which are consistent with the result of the 3H isotopic analyses. When recharging and transiting to geothermal water, the meteoric water underwent changes in geochemical compositions, which were dominantly controlled by water-rock interactions, including the precipitation of chalcedony and muscovite and the dissolution of calcite, fluorite, K-feldspar, and albite. In the Reshuizhou geothermal field, the hydrogeochemical compositions of the geothermal water are affected by cold water mixing.