In this paper, the flaky alumina (α-Al2O3) powder was prepared with using aluminum-air battery waste electrolyte as raw material, adjusting pH value with aluminum sulfate, and using potassium sulfate as molten salt. And the effects of synthesis temperature and crystal seed amount on the volume fraction, particle size, and particle size distribution of α-Al2O3 powder were studied. The structure and morphology of α-Al2O3 powder with different amounts of crystal seed were characterized by X-ray diffractometer, scanning electron microscopy, and laser particle size analyzer. The research results show that the volume fraction of α-Al2O3 powder increases, the particle size is reduced, the particle size distribution is more concentrated and the dispersity is better after adding crystal seed. In addition, when the crystal seed amount is 5%(mass fraction), the volume fraction of prepared flaky α-Al2O3 can reach 97.0%, and the average particle size is about 7.24 μm.