Abstract: The formation mechanism of geothermal salty water is important scientific basis for the exploitation of geothermal resources. The chemical components in geothermal salty water can reveal the experienced hydrochemistry, and the isotopic tracing can indicate the source of groundwater and hydrochemical processes. Therefore, water chemistry and isotopes were chosen to reveal the source and formation mechanism of geothermal salt water in Rudong geothermal field, Guangdong. The hydrochemisty indicates that the Rudong geothermal field develops high salinity geothermal water (TDS > 8000 mg/L) with Cl⁻ and Ca²⁺ contents as high as 5059.76 mg/L and 1991.42 mg/L, respectively, which is Cl-Ca·Na type water. The reservoir temperatures estimated with K−Mg geothermometer and chalcedony geothermometer are about 88.13~121.42℃, and the circulation depths are about 1963~2790 m. Hydrogeological conditions and hydrogen-oxygen isotope results confirm that the geothermal water comes from atmospheric precipitation recharge in the eastern hills with the recharge elevations of 260~315 m. Based on the ion ratio coefficient and ⁸⁷Sr/⁸⁶Sr, the environment for groundwater transport in the Rudong geothermal field is relatively closed, and the dissolution of granite minerals occurs more fully at high temperatures, while the dissolution of evaporative salt rock in the surrounding rocks provides the main water chemical components. The possibility of seawater intrusion is weak, and there is a certain cation exchange between the geothermal salty water and the surrounding rocks. Therefore, the dissolution of evaporative salt rock in the surrounding rocks and the dissolution of granite minerals are the main source of chemical composition for the geothermal salty water in Rudong geothermal field.