Abstract: Nanjing Tangquan is rich in geothermal water.It is of great significance for the sustainable use of geothermal water to reveal its supply source and genesis mechanism.The method of hydrochemistry and isotope geochemistry was taken to make the systematic study.The chemical composition of geothermal water is different from that of shallow cold water.The thermal reservoir temperature has been estimated to be 63~75℃, with a water circulation depth of 1.8~2.3 km.The geothermal fluid origins from the rainfall, and the altitude of recharge area is 321~539 m.The water age is 2046~6474 a, with shallow karst cold water mixed while upwelling, and the mixing ratio is 4%~26%.Based on these analysis, a genesis model of the study area has been postulated to be a low-medium temperature geothermal system of a convective type.It receives the precipitation from the karst outcropping area in Laoshan complex anticline, and it is gradually heated by the normal heat flow during circulating.The thermal reservoir is originally the Upper Sinian series dolomite.The cap rocks are mainly Cambrian, Cretaceous and Quaternary strata.The geothermal water flows upward in the intersection zones of the NEE- and NW-striking faults along deep circulation, and mixing with cold water in shallow, forming the geothermal anomalies.