In a catalytic reaction, after a reaction product leaves a catalyst bed, an inert substance with a low temperature is sprayed, and through heat absorption and vaporization processes of the inert substance, the temperature of the reaction product drops rapidly when staying in a catalyst cushion layer at a discharge end of a fixed bed reactor, or in a space formed by the catalyst cushion layer at the discharge end of the fixed bed reactor and a reactor head, or in a space formed by a tube plate at the discharge end of the fixed bed reactor and the reactor head. The residence time of the reaction product is shortened due to the entrance of the inert substance in a gaseous state.