The invention relates to a natural pharmaceutical extractive, and a preparation method thereof. The natural pharmaceutical extractive comprises concentrated extractive of water decocted radish seed, extractive of cortex phellodendri-vinegar naphtha, and sulforaphane according to the proportion of (6-9) : (3-6) : (3-5). The natural pharmaceutical extractive has an excellent curative effect on diarrhea related to cancer chemo-treatment and a synergistic effect on cancer chemo-treatment.