The invention discloses an oral drug for treating dysmenorrhea. The oral drug is prepared from the following active pharmaceutical ingredients in parts by weight: 12-15 parts of Sichuan lovage rhizome, 12-15 parts of combined spicebush root, 5-8 parts of frankincense, 15-20 parts of rhizoma corydalis, 12-15 parts of dried ginger, 15-20 parts of motherwort herb, 5-10 parts of liquorice, 8-10 parts of dried tangerine peel, 12-15 parts of Chinese yam, 10-12 parts of hawthorn and 10-15 parts of ass hide glue. A preparation method comprises the steps of taking the active pharmaceutical ingredients, adding water to immerse the active pharmaceutical ingredients, decocting the active pharmaceutical ingredients for 1-2 hours, filtering the decoction, then adding water to the dregs to immerse the dregs, decocting the dregs for 0.5-1.5 hours, filtering the decoction and merging the two filtrates, thus obtaining the oral drug. When used for treating dysmenorrhea, the oral drug is taken once a day, a dose once for seven days. One course of treatment lasts seven days. Generally, the oral drug takes an effect after being taken for two-three courses of treatment and cures dysmenorrhea after being taken for three-four courses of treatment. Through many years of clinical verifications, the oral drug for treating dysmenorrhea has good curative effects, takes an effect quickly, does not have toxic or side effects and is mainly used for treating primary dysmenorrhea.