The invention relates to a preparation method of prepared rehmannia root polysaccharide, which effectively resolves the problem that the prepared rehmannia root polysaccharide is extracted from the prepared rehmannia root polysaccharide. The technical proposal for the resolution is that water with 8 times of bulkfactor is added into the prepared rehmannia root, leaching is carried out for 4 hours at the temperature of 98 DEG C to 100 DEG C, filtration is carried out, water with 8 times of bulkfactor of original drug is added in dregs, leaching is carried out for 4 hours at the temperature of 98 DEG C to 100 DEG C, water in the dregs is squeezed completely, filtration is carried out, filtrates are combined and concentrated to 0.5g/mL of raw drugs, alcohol is added and mixed to lead alcohol content to reach 80 percent, slow addition and fast mixture are carried out, depositing in alcohol is carried out for 24 hours, suction filtration is carried out, 95 percent alcohol with 4 times of bulkfactor of the drugs is used for abstersion and sedimentation and volatized till no alcohol odor is left, water is added to dissolve to 2 times of bulkfactor of the crude drugs, pH is adjusted to 8 to 9, anhydrous calcium chloride is added to lead W/V to reach 7 percent, dissolution is carried out, room temperature is applied for the placement thereof, centrifugation is carried out, supernatant is taken for concentration to thick paste, and the prepared rehmannia root polysaccharide is obtained after drying at reduced pressure of is carried out at the temperature of 95 DEG C. The anhydrous calcium chloride deprives the protein of the prepared rehmannia root polysaccharide and avoids the use of poisonous reagent, therefore, the protein deprivation efficiency reaches 50.54 percent and the preservation ratio of the prepared rehmannia root polysaccharide reaches 77.15 percent. Therefore, the preparation method has enormous economic and social benefits.