The invention provides a technology method and a device for preparing ammonium diuranate. The technology is used for preparing ADU (ammonium diuranate) with high conversion activity from UO2F2 solution which is prepared by hydrolyzing UF6 and serves as a raw material by performing an ADU precipitation reaction of the UO2F2 solution and ammonia in a reactor. In the technological process, ADU slurry which is formed after prior batch of precipitation is completed is used as detained ADU slurry; and the detained ADU slurry is added into the UO2F2 solution to start to carry out next precipitation batch. The ADU prepared by adopting the technical scheme of the invention has high conversion activity and can be used for preparing UO2 powder with high sintering activity. The invention also designsa device for the ADU precipitation technology. By the device, the ADU preparation technology can be ensured to be carried out safely. Meanwhile, the technology method has the advantages that the technology method has short process and little equipment and automatic control operation is convenient to realize, etc.