Abstract: The ultrafine powder material is one of the key research fields of new materials. Ultrafine alumina is widely used in fine ceramics, composite materials, catalysts and their supporters due to its high strength and large specific surface area. The conventional methods of preparing ultrafine powders have many problems, such as large particle size distribution, high production cost, serious pollution and complex technology. However, the unique advantages of ultrasonic technology in controlling the particle size, particle size distribution and morphology of ultrafine powders have gradually become the frontier technology for ultrafine powders preparation. In this paper, aluminum hydroxide was used as raw material, and ultra-fine α-Al₂O₃ with a particle size of 100-200 nm was prepared by ultrasonic-assisted aluminum ammonium carbonate thermal decomposition method through acid dissolution of aluminum hydroxide, ultrasonic precipitation of aluminum salt, drying of precursor and final calcination. The products were characterized and analyzed, and the ideal process parameters for preparing ultrafine alumina with small particle size and good dispersion were explored.