The invention provides a preparation method of rhizoma polygonati. The preparation method comprises the following steps: (1) taking a rhizoma polygonati raw medicinal material, adding black bean juice, stirring uniformly, and performing covered moistening until the medicinal material is completely fermented; (2) taking the medicinal material after covered moistening in the step (1), performing high-pressure steaming for 1-3 times and 1-6 hours a time, wherein the temperature of the high-pressure steaming is 110-130 DEG C and the pressure is 0.048-0.172Mpa; and (3) after steaming, taking out the medicine, and drying to obtain a prepared product. The rhizoma polygonati prepared by the method provided by the invention is equivalent to even better than the rhizoma polygonati prepared by a conventional nine-steaming and nine-preparation process; meanwhile, the preparation process is simple, the production time can be shortened, human and material resources can be saved, clinical needs can be met, the conventional preparation process is inherited and promoted, and the preparation method provided by the invention has a good industrial application prospect.