The inversion methods of ground temperature field and ground temperature gradient field based on three-dimensional static heat conduction theory are presented with the influence from those minor factors such as heat convection and geology tectonics and so on. It can be used to simulate the three-dimensional ground temperature field in rock mass and to forecast the geothermal resources quantity of hot dry rock(HDR) on large scale. The paper analyzed the features and the mechanisms of ground temperature field and ground temperature gradient field of Rehai geothermal field in Tengchong，Yunnan province，China. Then the quantity of HDR geothermal resources was forecasted，and the results achieved from the presented simulation and forecast method of HDR geothermal resources of Rehai geothermal field can provide a reference to the geothermal exploitations.