The invention discloses a preparation method of a rare-earth-based composite multi-component denitrification and dioxin removal catalyst. According to the preparation method, titanium dioxide (titanium white) and silicon powder as carriers and ammonium metatungstate, ammonium metavanadate, cerous nitrate and lanthanum nitrate as active components, the steps of mixing, kneading, molding, drying and roasting are carried out in the presence of auxiliary materials to prepare the catalyst, wherein the auxiliary materials include monoethanolamine, citric acid, ammonium hydroxide, lactic acid, stearic acid, glass fibers, macromolecular polymer fibers RP-CHOP, hydroxypropyl methyl cellulose, polyoxyethylene and water. The prepared catalyst can simultaneously decompose nitric oxides and dioxin and can improve the mercury removal efficiency of the existing smoke control device. The prepared catalyst is not only applicable to new power plants, but also suitable for the modification of a wet-method desulphurization device of an old power plant, 80 to 95 percent of HgO generated in a coal burning power plant can be oxidized to Hg<2+> by utilizing the prepared catalyst, and a majority of particle-state Hg<2+> can be removed by utilizing a dust removal device.