The invention discloses a rare-earth-based catalyst achieving denitration, demercuration and dioxin removal simultaneously, a preparation method and application. Cerium oxide and erbium oxide are adopted as catalytic active components, a mixture of zirconium oxide and / or yttrium oxide and / or manganese oxide is adopted as a promoter, and titanium dioxide submicro spheres generated by optimizing andmodifying tetrabutyl titanate with hydrochloric acid are adopted as a main carrier. An active component and promoter precursor compound solution, the optimized and modified catalyst carrier and an organic forming additive are stirred to be uniform, mixed, aged and extrude-molded, drying and roasting are carried out, and the rare-earth-based catalyst is prepared. The catalyst is environmentally friendly, denitration, demercuration and dioxin removal can be carried out efficiently at the same time, the active temperature interval is wide, the mechanical strength is high, the oxidation efficiency of SO2 is low, and the cost is low. The denitration efficiency and the dioxin removal efficiency at the temperature of 220 DEGC to 400 DEG C are higher than 90%, and the demercuration efficiency ishigher than 95%. The rare-earth-based catalyst can be widely suitable for the fields of coal-fired power plants, chemical plants and the like.