The invention discloses a method for preparing alpha-isophorone. The method comprises the step of generating the condensation reaction by acetone under the alkaline catalysis to obtain the alpha-isophorone. The method is characterized in that the condensation reaction is performed in a water and dimethylbenzene mixed solvent system. According to the method, the condensation reaction is performed in the water and dimethylbenzene mixed solvent system; after the condensation reaction is performed for certain time, the selectivity of the alpha-isophorone is basically stabilized; even if the condensation reaction continues to be performed, the influence on the selectivity of the alpha-isophorone, mesityl oxide and isopropyl-ketol is very small; the phenomenon that a reaction endpoint is required to be controlled by the conventional synthesis method is avoided, and the method disclosed by the invention is suitable for industrial production. According to the method disclosed by the invention, the acetone conversion rate is 25-55 percent; the selectivity of the alpha-isophorone is 65-80 percent, and the total selectivity of the mesityl oxide and the isopropyl-ketol is 0.5-10 percent.