The invention relates to the technical fields of medicine, food and health products, and discloses a highly efficient preparation method of antitumor drug sodium cantharidinate. Add cantharidin essence into sodium hydroxide aqueous solution, heat to dissolve, add activated carbon to reflux for decolorization for 0.5-1 hour, heat filter, wash with water, concentrate under reduced pressure, add ethanol, cool to room temperature, precipitate solid and dry to obtain white powder. Wherein the equivalent ratio of cantharidin fines and sodium hydroxide is 1:2-3, and the weight ratio of cantharidin and ethanol is 1:3-10. The synthesis of sodium cantharidinate and the decolorization and refining of activated carbon are completed simultaneously, and the crystallization is carried out with ethanol-water system. The invention provides a synchronous method for synthesizing sodium cantharidate and decolorizing and refining activated carbon, the preparation process is simpler, the reaction cycle is shortened, the application of a large amount of acetone and isopropanol is omitted in the reaction process, the production cost is reduced, and The pollution to the environment is small, the quality control is simpler, the production efficiency is improved, and the industrial production is convenient.