Abstract <p>In central and eastern Inner Mongolia, specifically in the Xilinhot and Chifeng areas, multiple tectonic movements have taken place and strong Cenozoic volcanic activities have occurred, endowing the region with abundant medium and high-temperature geothermal resources. Identifying the sources and causes of geothermal water in this area can help to predict the distribution of geothermal resources and provide a reference for the development of geothermal energy here. Therefore, this paper investigates the geochemical genesis and formation mechanism of geothermal water in the target area through major ions and trace element analysis, as well as hydrogen and oxygen isotope analysis. The results show that the geothermal water in the Xilinhot area is HCO₃–Na and HCO₃–Cl–Na types, dominated by fully equilibrated water and partially equilibrated and mixed water. In the Chifeng area, the geothermal water type is SO₄–Na, mainly consisting of partially equilibrated and mixed and immature water. The anomalously high values of trace elements such as B, Li, Rb, and Cs are fixed in both areas. Moerover, their correlation with Cl^– indicate that the geothermal water in this region is generally influenced by deep parent geothermal fluids (mantle magmas). The δD–δ¹⁸O relationship reveals that the Chifeng geothermal water is shallowly buried, mixed with shallow cold water, and influenced by precipitation; whereas the Xilinhot geothermal water is mainly derived from precipitation and magmatic water. Finally, two geothermal water genesis models, namely the Xilinhot-type and Chifeng-type, are established. This study uncovers the evolutionary process of deep geothermal water circulation in the target area, which is of positive significance for the development of geothermal resources in the region.</p>