A method for preparing diazoxide includes reacting o-aminobenzenesulfonamide with N-chlorosuccinimide in a chlorine solvent to obtain 2-amino-5-chlorobenzenesulfonamide, mixing the 2-amino-5-chlorobenzenesulfonamide, an imidazole salt and an amide solvent, then heating same for reaction so as to obtain diazoxide; or mixing o-aminobenzenesulfonamide, an imidazole salt and an amide solvent, then heating same for reaction to obtain a compound IV; then reacting the compound IV with N-chlorosuccinimide in a chlorine solvent to obtain diazoxide. The application of imidazole hydrochloride as a catalyst in preparing diazoxide is also disclosed. The present invention avoids the use of highly corrosive and toxic chlorosulfonyl isocyanate, a strong acid (sulfuric acid), and a high reaction temperature (240-250° C.), and the reaction steps are short; the total yield of the two steps is more than 90%, and compared with publicly disclosed preparation methods for diazoxide, the synthesis route overcomes numerous shortcomings, thus being more suitable for industrial production.