The invention relates to the technical field of a device for preparing magnesium sulfate by waste sulfuric acid, and particularly relates to a method for preparing magnesium sulfate by alkylate waste sulfuric acid and a device for preparing magnesium sulfate by alkylate waste sulfuric acid. The method includes steps of performing material mixing, high-temperature calcination, primary cooling and dust removal, secondary cooling and dust removal, and tail gas treatment and other processes on magnesium oxide and waste oxide, thus the method and device for preparing magnesium sulfate by alkylate waste sulfuric acid have good effect in treating high-concentration organic matter waste acid treatment and not easy to generate residues.