The invention discloses a method for preparing vanadium pentoxide by treating a vanadium-containing waste sulfuric acid catalyst through a two-step method. The pH value is controlled to be 5-6 throughoxalic acid leaching in a first step, namely the leaching rate of vanadium is controlled to be between 65% and 75%, and simultaneously a large amount of impurities (Al, Ni, Cu and the like) are inhibited from being separated out; the PH is controlled to be 1.5-2.0 through sulfuric acid leaching in a second step, the leaching accumulation of vanadium is controlled to be larger than or equal to 90%, meanwhile, a small amount of impurities such as Al are leached, leaching liquid returns to the first step of oxalic acid leaching (when the small amount of impurities leached in the second step return to the first step of leaching, precipitates are generated again and enter slag), and meanwhile, the first-step leaching is completed by utilizing the temperature of the leaching liquid, so that thecost is reduced; leaching residues are washed in the second step, the leaching residues are centrally collected, lime is added to be mixed, and the mixture is sold to a cement plant to serve as clinker of the cement plant; the leaching liquid is prepared after the first step of oxalic acid leaching, vanadium precipitation of ammonium sulfate is directly adopted to prepare an ammonium polyvanadateintermediate product, concentrated sulfuric acid needs to be added in the production process, residual oxalic acid in the leaching liquid completely reacts with the concentrated sulfuric acid to eliminate obstacles for subsequent water treatment, and solid ammonium sulfate prepared after water treatment returns to a production system, and potassium sulfate is purified and sold to potash fertilizer production enterprises to be used as a raw material.