Disclosed are a method for preparing a pure isophthalaldehyde bisulfite adduct free from impurities through a specific purification process, and use thereof as a starting material for polymerizing polybenzimidazole under a mild condition. According to the present disclosure, it is possible to obtain a pure isophthalaldehyde bisulfite adduct free from impurities, such as unreacted materials or byproducts. In addition, it is possible to accomplish industrial mass production of a high-molecular weight polybenzimidazole by using the adduct as a starting material for polymerizing polybenzimidazole under a mild condition in an organic solvent.