<p>Geothermal heat in southern Jiangxi is distributed along NE–NW-trending faults, and its genetic mechanism is the combined result of deep circulation heating of atmospheric precipitation and structural–lithological coupling. Through hydrochemical analysis (Piper three-line diagram, Spearman correlation), hydrogen and oxygen isotope tracing (δD–δ¹⁸O, recharge elevation), and heat reservoir temperature calculation and simulation (quartz temperature scale, circulation depth), this study found the following. The Quannan–Xunwu fault zone is characterized by HCO₃^-–Na-type high-temperature heat reservoir (98–155&#xa0;℃), reflecting the radioactive heat generation of Yanshanian granite (3.08&#xa0;μW/m³), and deep-circulating silicate dissolution and filtration are dominant. The HCO₃^-–Ca type of water (55–130&#xa0;℃) in the Shicheng–Xunwu fault zone is due to the coupling of limestone dissolution and structural permeability, and this is a fault zone convection-type geothermal. The Yingtan–Anyuan fault zone is characterized by SO₄^2-–Ca type of water (60–120&#xa0;℃), which is controlled by the oxidation of sulfide minerals (pyrite FeS₂) in the fault zone and the mixing of shallow cold water. The HCO_3-–Ca–Mg type of hydrothermal fluid (70–140&#xa0;℃) in the Dayu–Nancheng fault zone reflects the transition characteristics of plagioclase weathering and dolomite dissolution. The Na–Cl type of hydrothermal fluid (60–142&#xa0;℃) in the Ganjiang fault zone indicates mixing of deep brine. The residence time of geothermal water is 4712–36,727&#xa0;years, and the runoff rate is 0.04–1.17&#xa0;m/a, reflecting slow and deep circulation characteristics. The research results provide key scientific basis for the exploration and cascade development of granite geothermal resources in South China.</p>