The invention relates to an externally applied preparation for treating whelks. The externally applied preparation is prepared by dissolving one or several medicaments of erythrocin, aureomycin, chloramphenicol, lincomycin hydrochloride, tobramycin, natamycin, norfloxacin, rifampicin, cyclosporine, gentamicin and neomycin into a polysaccharide aqueous solution. An obtained solution contains 0.005-1 wt% of medicaments and 0.01-30 wt% of glucan, xyloglucan, mycose, sea-tangle polysaccharides, tamarind gum, pectin or other Chinese medicinal herb polysaccharides, or a proper quantity of glucose, sodium chloride and potassium chloride are added to the aqueous solution to prepare a plaster preparation. The externally applied preparation has quick effect and definite curative effect on the whelks or acne and has small stimulation to the skin, extremely light skin decrustation degree and no obvious toxic or side effects. The process of the preparation can guarantee the chemical stability of each component in a production process.