The method of multiphase synthesis of NH₄Hf₂F₉ for accelerator mass spectrometry (AMS) measurement was researched. The structure and composition of NH₄Hf₂F₉ powder were characterized by XRD, SEM, XPS and TG-DTA. When the NH₄Hf₂F₉ powder was heated at 573 K for 1 h, the powder phase was still NH₄Hf₂F₉. After being heated at 773, 973, 1173 and 1373 K for 1 h, respectively, they all transformed to HfO₂. When the temperature was above 573 K, NH₄Hf₂F₉ powder began to decompose. When the temperature reached 1073 K, the weight loss of NH₄Hf₂F₉ powder was about 78%. The powder shape was irregular, and powder size was between 5 and 20 μm in the SEM images. The XPS results show that there were Hf, N and Hf in NH₄Hf₂F₉ powder, and Hf mainly existed in quadrivalent state (+4).