Abstract: As a critical tracer for deep fluid activities, gas isotopes hold significant scientific significance for elucidating the formation mechanism of fault-controlled geothermal resources. The genesis of geothermal resources in the Wentang fault zone is jointly governed by fault structures and heat sources; however, the underlying heat source mechanism remains to be further investigated. In this study, six geothermal fluid samples were collected from the Wentang fault zone, followed by systematic analysis of the gas composition and helium (He) isotope signatures of dissolved gases in geothermal water. Integrating the He isotopic characteristics with terrestrial heat flow estimation, the heat source contribution and thermal structure characteristics of the fault-controlled geothermal system were revealed. The results demonstrate that: (1) Dissolved gases in geothermal water are dominated by N₂ and O₂, with concentrations ranging from 88.43% to 96.46% and 1.23% to 9.19%, respectively; the ³He/⁴He ratio varies between 0.08R_a and 0.82R_a, indicating that helium is primarily derived from the mixture of crustal radioactive decay and atmospheric input, with negligible mantle-derived contribution. (2) Terrestrial heat flow values estimated from the corrected He isotope ratios range from 34.47 to 62.00mW/m², with an average of 47.53mW/m², exhibiting a “hot crust-cold mantle” lithospheric thermal structure. (3) A significant negative correlation exists between the cold water mixing ratio and heat flow values (R²=0.84), suggesting that the dilution effect of shallow cold water notably impacts the accuracy of terrestrial heat flow estimation. By integrating the isotopic characteristics of dissolved gases in geothermal water, this study further confirms that the heat source of geothermal resources in the Wentang fault zone is predominantly crust-derived, providing an isotopic geochemical foundation for geothermal exploration and geothermal reservoir assessment in the study area.