The invention discloses a method for preparing alpha-alumina powder for liquid crystal substrate glass, which is prepared from the following raw materials in part: 90 to 99 parts of first class industrial alumina or/and first class industrial aluminium hydroxide, 0.3 to 5 parts of boric acid, 0.2 to 5 parts of ammonium chloride and 0.2 to 2 parts of barium fluoride. The method comprises the following steps: firstly, mixing the raw materials uniformly, placing the mixture in a tunnel kiln, calcining the mixture at the calcination temperature of 1450 DEG C and keeping the temperature for 6 hours; secondly, sorting and grading the quality of a product of the first step, crushing, grinding and grading the product by placing the product in a continuous ball-grinding grading production line to obtain alumina powder with the required grain size distribution; and thirdly, putting a product of the second step into purified water, washing the product to make a pH value of the alumina powder between 7.5 and 8.5, and drying the product to obtain the alpha-alumina powder. The alpha-alumina powder for the liquid crystal substrate glass prepared by the method has the following specific indexes: Al2O3 content of more than or equal to 99.8 percent, alpha-phase transformation rate of more than or equal to 98 percent, SiO2 content of less than or equal to 0.03 percent, Fe2O3 content of less than or equal to 0.02 percent, Na2O content of less than or equal to 0.02 percent, C content of less than or equal to 0.01 percent, Cl<-> content of less than or equal to 0.001 percent, SO3 content of less than or equal to 0.01 percent, ignition loss of less than or equal to 0.01 percent and alpha-alumina grain size of between 30 and 75 micrometers.