The invention discloses a medicine for treating skin diseases, which consists of radix cynanchi atrati, aloe, cortex meliae, alum, spina gleditsiae, fructus bruceae, ginger, pepper, purple perilla, radix angelicae, fructus xanthii, herba menthae, cortex phellodendri, radix sophorae flavescentis, radix arnebiae, sargentgloryvine stem, cortex dictamni, cortex lycii radicis, radix cynanchi paniculati, fructus kochiae, fructus cnidii, safflower, fructus tribuli, flos chrysanthemi indici, radix stemonae, fructus psoraleae and clove according to a certain weight ratio. A preparation method of the medicine comprises the following steps: adding the components into a bottle according to the weight ratio and adding alcohol with mass concentration of 75%, and soaking for 7 days; the liquid medicine can be directly applied to the wound and also can be diluted with warm water; and the process is simple and easy to obtain and convenient to use. The medicine disclosed by the invention has the functions of drying dampness and relieving itching, stopping pain, sterilizing, killing worms and removing toxin, activating blood and resolving stasis, dispelling wind and releasing exterior, killing lice and worms, clearing heat and removing toxin and deeply killing fungi, takes effect very quickly and immediately, eliminates the pain of the patients, and can cure multiple skin diseases while overcoming the shortcoming of slow effect of orally taking traditional Chinese medicine.