The invention relates to the technical field of extraction and preparation of manninotriose, in particular to a method for extracting manninotriose from fresh radix rehmanniae. The method comprises the steps as follows: high-temperature processing of the fresh radix rehmanniae; preparation of a manninotriose sample solution; alcohol precipitation; impurity removal, decoloration and concentration; chromatographic separation. The fresh radix rehmanniae is subjected to high-temperature processing directly without operation on raw radix rehmanniae, processing steps of the fresh radix rehmanniae are greatly reduced, and positive influence is realized from the yield data; the temperature of water extraction is 70-80 DEG C, rather than frequently used boiling water for extraction, and a significant impact on increasing of the yield is realized; according to the manninotriose prepared through resin and gel in combination after high-temperature steaming, the total yield is 4.0% or higher, the purity is higher than 98.0%, and an environment-friendly, convenient and economic method is provided for preparation of the high-purity manninotriose.