Abstract: Abstract: Eighty water samples were collected from different geothermal fields in Beijing and the following five kinds of characteristics of deuterium excess parameters were analyzed: (1) The average d value of geothermal water is 5.40, the average d value of cold groundwater is 6.04, which is bigger than that of the geothermal water, so the oxygen isotope exchange caused by water?rock interaction is easier in geothermal water than in cold groundwater. (2) The oxygen isotope has remarkable isotope exchange process, and the d value increases with the age of the geothermal groundwater. When the age of the water is (12.76±0.13) ka, the d value is 11.2; when the age of the water is (38.96±0.63) ka, the d value is 14.6. (3) The d value decreases with the depth of the geothermal groundwater in the same area; when the depth is 3221 m, the d value is 3.03; when the depth is 125.13 m, the d value is 5.72. (4) The d value decreases from the recharge area to the discharge area. The average d value is 7.31 in northern recharge area, 5.68 in Beijing depression, and ?9.20 at the southern edge of Beijing plain. The velocity of flow decreases with the difference d values. (5) The d value increases with the redox potential when it is smaller than 200 mV. When the redox potential is ?326 mV, the d value is ?9.20; when the redox potential is 158 mV, the d value is 7.48. The d value decreases with the redox potential when it is bigger than 200 mV.