Beijing Olympic Park Area is a newly discovered geothermal resource area in Beijing. Based on the geochemistry, H-O isotopes, tritium isotopes and 14C dating data of the geothermal fluid and combining the geothermal geology background, the authors studied the characteristics and the formation mechanism of the geothermal fluid in this area. The major hydrochemical type of the geothermal fluid is the type of HCO3--SO42--Cl--Na+, and the minor one is HCO3--NO3--Ca2+-Mg2+ type. The geothermal fluid was not supplied by the meteoric water and the runoff of the geothermal water is controlled by the regional fault system. The geothermal water is a mixed type of geothermal water between original geothermal water and deep cold underground water, and the original geothermal water was formed in 20 000~30 000 a B.P. This area holds a big potential for geothermal water supply.
