Abstract: In order to preventing water from commixing heavy water, the pipeline and equipments of the heavy water system must be dewatered before filling heavy water. The dewatering technology of combining hot air blowing method with vacuum-drying method was applied to dry the pipeline. Heating compressed air and local heating with electric heating belt were adapted, and the water dew point at the system outlet was detected, the maintaining vacuum test was performed in the drying process to assure the drying degree of the system meet the specification. The heavy water concentration filled in the drying system was reduced to be less than 0.02%, and it indicates this dewatering technology is effective for drying heavy water system of nuclear reactor.