Disclosed is a method for preparing the same. The method for preparing a carrier including ammonium oxidizing bacteria immobilized therein includes: preparing a PVA-alginate mixed solution containing PVA mixed with alginate; adding sludge containing ammonium oxidizing bacteria and sodium bicarbonate (NaHCO3) to the PVA-alginate mixed solution to obtain a foaming-beading solution; and dropping the foaming-beading solution to a saturated boric acid solution to obtain beads including sludge immobilized therein, wherein sodium bicarbonate (NaHCO3) is decomposed to produce carbon dioxide (CO2) which is discharged to the exterior of the beads to form pores in the beads, when the foaming-beading solution is dropped to the saturated boric acid solution to obtain beads including sludge immobilized therein.