The present invention discloses a sulfonamide compound (III) which is intermediately produced in a process for preparing a high-purity sulfonamide compound (I). The preparation comprises the following steps: a, taking a crude product of a sulfonamide compound (I) as an initial raw material, and enabling the raw material to react with a compound of a formula (II) in presence of alkali and a catalyst so as to synthesize an intermediate of a formula (III); and b, enabling the compound represented by the formula (III) to react with alkali or acid, thereby obtaining the high-purity sulfonamide compound (I).