The invention discloses a method for preparing battery-grade manganese sulfate from alkylation waste sulfuric acid. The method comprises the following steps: 1) purifying the alkylation waste sulfuric acid; 2) grinding manganese ore, adding sulfuric acid for leaching, and obtaining a leachate; 3) regulating pH of the leachate, leaving the leachate to stand and filtering the leachate, and taking a filtrate A for standby application; 4) adding manganese(II) fluoride to the filtrate A, leaving the solution to stand and filtering the solution, and taking a filtrate B for standby application; 5) adding sodium sulfide to the filtrate B, leaving the solution to stand and filtering the solution, and taking a filtrate C for standby application; 6) adding sodium carbonate to the filtrate C, leaving the solution to stand and filtering the solution, taking, washing and drying filter residues to obtain manganese carbonate powder; 7) dissolving the manganese carbonate powder in sulfuric acid, adding ethanol, radiating the solution with ultraviolet light of 315-400 nm and 0.5-0.8 wm<2>, filtering the solution, and washing and drying filter residues to obtain battery-grade manganese sulfate crystals. The produced manganese sulfate is high in purity, and the production cost and environmental pollution are reduced.