The invention discloses a method for preparing alpha-isophorone, which comprises the following steps: pressurizing a mixture of acetone and recycled acetone to 8.0-20.0MPa under the condition of continuous steady state operation and preheating to 280 DEG C-320 DEG C; then mixing the mixture with 10 percent of NaOH solution or 10 percent of KOH solution pressurized to the same pressure; carrying out super-critical reaction for 1-3 min in a pipe type reaction vessel; decompressing the reaction liquid to 3.0-4.0MPa and entering a flashing steam tower; obtaining the recycled acetone mixture from the top of the tower; leading tower kettle liquid to enter a hydrolyzing tower to hydrolyze polymers (C12 and C15); continuously extracting hydrolyzed reaction liquid from a tower kettle of the hydrolyzing tower, cooling, decompressing to normal pressure and layering in a layering device to obtain an alpha-isophorone rough product layer and a byproduct water layer. The method for synthesizing the alpha-isophorone has short needed reaction time, few byproducts and high reaction yield and is suitable for industrialized production.