This paper presents an alternative computationally efficient approach to the thermal design of high-power DC reactor which is applied to ITER poloidal field converter module. The proposed approach takes full advantages of the property of time-saving from heat transfer theory and high accuracy from 3-D finite-element analysis, focusing on the predictions of the temperature rise and pressure loss of circulating water. Thermal measurements from two prototypes test show a good agreement with the predictions, which proves the high efficiency and accuracy of the proposed approach in the design of hundred-kilowatt-class thermal power DC reactor.