The invention provides a method for preparing a medicine for treating psoriasis, and aims to solve the problems of difficult treatment, easy relapse and the like of psoriasis. The method comprises the following steps: firstly, washing and soaking kelp, rheum officinale and Chinese chive roots to obtain a residual liquid A; then, soaking garlic, pepper and rust into mature vinegar, heating and boiling to obtain a filtrate B; and finally, uniformly mixing the residual liquid A and the filtrate B to obtain a medicinal solution. The medicinal solution is simple in preparation method and low in cost, and the raw materials are wide in source and reasonable in compatibility and are synergistic, so that the medicinal solution can be used for dispelling wind, eliminating dampness, promoting blood circulation to relieve pain, relieving itching, removing psoriasis, resisting bacteria, diminishing inflammation, enhancing immunity and promoting a skin lesion to quickly repair, has a good curative effect and a quick response when being used for treating psoriasis to achieve the effect of treating both symptoms and root causes, and can be used for achieving the target of preventing relapse after healing. The medicinal solution has a total effective rate of over 98 percent.