Study on the Nuanquanzi geothermal field in the Yanshan uplift is of great significance for understanding the origin of geothermal fluid in the intracontinental orogenic belt of the fault depression basin margin in North China. The geochemical characteristics and formation mechanism of the Nuanquanzi geothermal system were elucidated by classical hydrogeochemical analysis, multi-isotopes approach (δD, δ18O, δ13C, δ87Sr/86Sr), 14CAMS dating, and integrated geophysical prospecting of surface-soil radon gas measurement and CSAMT inversion. The results show that the Nuanquanzi geothermal field is a medium-low temperature convection-fault semi-enclosed geothermal system. The hydrochemical type of thermal water is primarily HCO3-Na, and rich in soluble SiO2, F− and Cl−. The geothermal water primarily originated from the recharging meteoric water with a maximum circulation depth of 2 400–3 200 m, but affected by the mixing of endogenous sedimentary water. The reservoir temperature calculated by Na-K and quartz geothermometer of the Nuanquanzi geothermal system was determined to be 73.39–92.87 °C. The conduction-cooling and shallow cold-water mixing processes occurred during the parent geothermal fluid ascent to surface, and the proportion of cold-water mixing during circulation was approximately 88.3% to 92.2%. The high-anomaly radon zones matched well to the low apparent-resistance areas and hiding faults, indicating that the Nuanquanzi geothermal field was dominated by a graben basin restricted by multiple faults.