The invention relates to the medical field, in particular to a drug for treating asthma and a preparation method thereof. The drug comprises grapefruit pulp, grapefruit peel, nidus vespae, orange pulp, rosa roxburghii Tratt., rhizoma belamcandae, bulbus fritillariae cirrhosae, olive bark, pitaya peel, peanuts, grosvener siraitia, white turnip, sinapis semen, periostracum cicadae, flos farfarae, climbing groundsel herb, pheretima and a proper amount of deionized water. The drug prepared by using the method is obvious in curative effect and small in side effect and not likely to produce drug resistance; the bulbus fritillariae cirrhosae, the olive bark and the pitaya peel are added into the raw materials, so that the drug has the functions of clearing away the heat and decreasing the internal heat, and the curative effect of drug is improved; the white turnip, the sinapis semen, the periostracum cicadae, the flos farfarae, the climbing groundsel herb and the pheretima are added into theraw materials, so that the drug has the performance of clearing the lung and eliminating phlegm, and the effect on asthma treatment is obviously improved; in the preparation method, extraction is conducted with absolute ethyl alcohol, the effective components of the drug are completely extracted, and the drug properties are higher; meanwhile, sterilization is conducted by using a high-pressure sterilization pot, so that there are no bacteria in the finished drug, bacterial infection is prevented, and the drug effect is easily maintained.