The invention discloses a method for preparing a flue gas desulfurization powder adsorbent from liquid-solid wastes, which comprises the following steps of: performing acidolysis on ferromanganese solid wastes by using copper electrolysis wastewater, and separating to obtain a sulfate solution; then removing sulfate ions in the sulfate solution by using lime milk and acid, and separating to obtain a metal salt solution; adding carbide slag into the metal salt solution for precipitation reaction, standing for aging, and separating to obtain a gelatinous product; and finally, drying, calcining, crushing and finely grinding the gel to obtain the desulfurization powder adsorbent. The active components of the desulfurization powder adsorbent prepared by the method are CaO and MnO, and the auxiliaries are Fe3O4 and ZnO; the catalyst contains catalytic active components CuO and NiO, and can catalyze SO2 and O2 in the flue gas to react to generate SO3; the desulfurization powder adsorbent is small in particle size, complete in micropores, large in pore volume and high in desulfurization activity; and the method is suitable for dry dedusting device desulfurization or flue gas spray drying desulfurization, does not need a desulfurization packed tower, and reduces the investment cost of enterprises.