The invention provides a preparation method of highly pure alpha-aluminum oxide, belonging to the technical field of metal organic compound synthesis. The preparation method comprises the following steps of making metal aluminum having purity of more than 99.5% react with mixed alcohol to prepare aluminum alkoxy, and then hydrolyzing, aging and drying the aluminum alkoxy to obtain highly pure alpha-aluminum oxide. The invention overcomes the purification difficulty when the aluminum alkoxy is prepared by low-carbon alcohol and the shortages of low reaction efficiency when the aluminum alkoxy is prepared by high-carbon alcohol; the alpha-aluminum oxide with the purity as high as 99.9% can be obtained by controlling the mixing alcohol proportion and the hydrolysis condition; and the invention simplifies the preparation technique of the highly pure alpha-aluminum oxide and has high preparation efficiency and low preparation cost.