The invention relates to a preparation method of a slimming drug which is prepared from the following Chinese medicine raw materials: raw rhubarb, ginger-processed coptis, raw hawthorn, fermentum rubrum, pummelo peel, lightyellow sophora root, rhizoma atractylodis, semen descurainiae, plantain seed and cassia seed. The preparation method comprises steps of decocting with water, and precipitating with alcohol.