The invention relates to a medicament for treating psoriasis vulgaris and a preparation method thereof. The medicament provided by the invention comprises the following raw materials in parts by weight: 30-45 parts of glabrous greenbrier rhizome, 20-30 parts of smilax, 9-12 parts of red peony root, 10-15 parts of safflower, 15-30 parts of sophora japonica, 10-15 parts of wild chrysanthemum flower, 10-15 parts of spina gleditsiae, 30-45 parts of dandelion, 10-15 parts of forsythia, 6-10 parts of scorpion, 20-30 parts of honeysuckle, 9-15 parts of radix rehmanniae, 9-12 parts of angelica sinensis, 15-20 parts of red sage root and 20-30 parts of dittany bark. The preparation method comprises the following steps: a, weighing all raw materials according to the parts by weight; b, putting the weighed raw materials into a medicine pot and adding water for soaking; c, boiling with strong fire and then decocting for 20-40 minutes with slow fire, filtering the decoction; and d, adding water to the medicine pot, boiling with strong fire and then decocting for 20-30 minutes with slow fire, filtering the decoction and mixing evenly with the decoction filtered for the first time. The medicamentprovided by the invention has very good curative effects on patients with psoriasis vulgaris at the active stage and the resting stage, and is safe and efficient without side effects.