The invention discloses a method for preparing alpha-imatinib mesylate, and belongs to the field of medicines. The method comprises the following steps of: suspending imatinib base in a mixed solvent of water and an organic solvent, stirring, and adjusting the temperature to be 20-30DEG C; dissolving methanesulfonic acid, and dripping into reaction liquid; after the dripping is finished, standingfor clarifying the liquid, heating to the temperature of between 50 and 60DEG C, and keeping the temperature to react for 0.5 to 1 hour; and reducing the temperature to 32-40DEG C, adding alpha-imatinib mesylate seed crystals, dripping the organic solvent, continuously reducing the temperature to 20-30DEG C, separating out crystals, filtering, and drying to obtain a product. The preparation method has the advantages of simple operation, short reaction time, high yield, high purity of alpha-imatinib mesylate, low production cost, and suitability for industrial production.