Al₂O₃ ceramics are the most widely used structural ceramics because of excellent mechanical properties, acid and alkali corrosion resistance and high quality-price-ratio. Due to serious agglomeration and high content of sodium and potassium impurities in domestic calcined Al₂O₃ powder, the preparation of Al₂O₃ ceramics with high performance relies heavily on imported powder. In this work, the Al₂O₃ powder produced by a domestic company was firstly disaggregated using ball milling, and the median particle size D₅₀ of the powder was reduced from 1.88 μm to 0.90 μm. Afterwards, the impurity content in the powder was reduced by washing with water, and the Na content was reduced from 140×10^-6 to 60×10^-6, which was close to the level of the same grade powder abroad. Finally, Al₂O₃ ceramic green bodies were prepared by spontaneous coagulation casting using MgO and MgAl₂O₄ as sintering aids. It is found that the addition of MgO increases the slurry viscosity, while MgAl₂O₄ does not affect that, and both can effectively reduce the sintering temperature and inhibit the grain growth. And the average grain size is reduced from 41.10 μm to 10.64 μm, which improves the mechanical properties of Al₂O₃ ceramics. Through disaggregation and purification, the domestic Al₂O₃ powder can reach the same level as the Japanese Sumitomo Al₂O₃ powder with the best performance in the same grade.