The invention provides a preparation method of modified cyclodextrin, and relates to the technical field of composite nature polymeric flocculants. The preparation method comprises the following stepsthat acrylamide-beta-cyclodextrin is provided; the obtained acrylamide-beta-cyclodextrin, dimethyl diallyl ammonium chloride, a silane reagent, ammonium persulfate and sodium hydrogen sulfite are mixed with water to generate a free radical polymerization reaction, and the modified cyclodextrin is obtained. The specific surface area of the modified cyclodextrin provided by the invention is 300-450m<2>/g, and the number of adsorption sites is increased. According to the data of the embodiment, under an optimal condition, the decolourization ratio of the modified cyclodextrin on printing wastewater can be as high as 97.24%; and the adsorption rate on chromium-containing sewage with the concentration being 800 mg/L can be as high as 95% after 30 minutes.