The invention discloses a method for preparing aluminum nitride by carbothermal nitridation-reduction of aluminum oxide under vacuum conditions, and relates to the technical field of vacuum metallurgy. The method comprises the following steps of (1) uniformly mixing aluminum oxide, carbon powder and water into a mixed material, and tabletting the mixed material at the pressure of 2 to 5MPa; (2), putting the material after being subjected to tabletting in step (1) into a crucible, closing a vacuum furnace, leading the nitrogen when the vacuum degree is 1Pa or lower, controlling the vacuum degree within 10 to 50 Pa, heating to 1550 to 1650 DEG C, and warming for 30 to 120min, so as to obtain a mixture of aluminum nitride and excessive carbon; (3) decarbonizing the obtained mixture in step (2), so as to obtain aluminum oxide powder. The method has the advantages that the required temperature in the preparation process is obviously lower, the time is short, and the demand amount of nitrogen gas is small; compared with the two-section preparation method of aluminum nitride, the industrial process is shortened, the infrastructure investment and production cost are reduced, and the industrialization is easier.