The invention relates to an external preparation for treating seborrheic dermatitis, comprising at least one effective parasite-expelling medicament which is extracted from etreptomyces and used as an active component, i.e. large ring lactone, such as abamectin, or chemical modified or synthetic derivative ivermectin, one or more of bactericidal antibiotics in erythrocin, aureomycin, chloromycetin, lincomycin hydrochloride, tobramycin, natamycin, norfloxacin, rifampicin, cyclosporine, gentamicin and neomycin and at least another traditional Chinese medical herbal amylase which are contained in carriers for the surface administration and the cutaneous penetration of the active component, or suitable amounts of glucose, sodium chloride or potassium chloride can be also added to the external preparation. The invention has rapid constringency and demanded efficacy for the seborrheic dermatitis, little excitation irritation to the skin, extremely mild skin peeling without obvious toxic and side effects; and the preparation process can ensure the chemical stabilities of various contents in the production process. The invention can be also used for treating whelks.