The invention discloses a novel external drug formula for treating acne and a preparation method of the formula. The formula comprises raw materials as follows: metronidazole, lincomycin, cimetidine and normal saline. The drug formula for treating acne is directly applied to skin, a patient does not need to take medicines or have an injection, and side effects caused by medicines during conventional application are reduced; the acne cure efficiency is higher, and the formula is convenient to use. The invention also provides a cimetidine crystal. The formula prepared from the cimetidine crystalhas better stability, is not prone to deterioration during long-time storage and has a better effect on treating the acne. The preparation method of the drug formula is simple, materials are easy toobtain and the cost is low.