Abstract: Adding poly acrylic acid (PAA) in the preparation of ammonium diuranate (ADU) was investigated. The experiment results show that adding a little amount PAA to the ammonium hydroxide can increase the sedimentation rate and improve the filtrate rate of ADU serosity evidently. The dried powder of ADU is incompact, and the UO₂ powder produced by ADU has excellent capability in pressing and sintering.