PURPOSE: A method for preparing o-sulfobenzimide is provided to prevent loss of effective ingredient during preparation process and massively prepare the o-sulfobenzimide at high purity. CONSTITUTION: A method for preparing o-chlorobenzimide comprises: a step of reacting o-chlorobenzoic acid with sulfonating agent under the presence of copper catalyst to convert chlorine atom of o-chlorobenzoic acid with sulfonate; a step of reacting the o-sulfobenzoic acid with alcohol under the presence of acid catalyst to perform alkylesterification of carboxy group of o-sulfobenzoic acid; a step of reacting the o-sulfobenzoic acid alkyl ester with chlorinating agent under the presence of inactivating organic solvent; and a step of reacting o-sulfochloride benzoic acid alkylester with ammonia for substituting chlorine atom with amino group and cyclizing to obtain o-sulfobenzimide.
