The invention provides an external medicine for treating a skin disease and belongs to the field of traditional Chinese medicines. The medicine is prepared from the following raw materials: cacumen biotae, peach leaves, folium artemisiae argyi, beautiful sweetgum fruits, senecio scandens, cortex phellodendri, cortex lycii radices, radix euphorbiae lantu, cortex dictamni, honeysuckle stem, wild chrysanthemum flowers, spina gleditsiae, Chinese honeylocust fruits, alum, belvedere fruits, fructus cnidii, brucea javanica, golden larch bark, hibiscus syriacus root and bark, rhubarb roots, garden balsam, ligusticum wallichii, leeches, peach kernels, sappanwood, polygala japonica, aizoon stonecrop herb, drynaria total flavonoids, herba leonuri, ground beeltle, sedum uizoon, pubescent holly roots, flos carthami, chloranthus serratus, asiatic toddalia roots and black vinegar. The external medicine has a good effect on various red and swollen skins, erosion, psoriasis, whelks, acnes, eczema and mosquito bites, and especially has an excellent effect on psoriasis. Meanwhile, the medicine is short in treatment course, fast to take effect, low in cost, convenient to use, free of side effect and not easy to recur.