A pharmaceutic composition for treating an allergic skin disorder. It has an active component and a carrier. The active component has a major active ingredient osthol and a group of minor active ingredients: xanthotoxol, xanthotoxin, isopimpinellin, bergapten, and imperatorine. The major active ingredient osthol accounts for at least 90% by weight of the active component, which accounts for 5-35% by weight of the overall pharmaceutic composition.
