The invention relates to a photochemical hydrogen emission reaction of a dihydro benzimidazole compound, in particular to a method for preparing a benzimidazole cationic compound and hydrogen from the dihydro benzimidazole compound by using a photochemical method. By means of the method, a corresponding benzimidazole cationic compound and hydrogen can be quickly prepared from the dihydro benzimidazole compound by a photochemical reaction of the dihydro benzimidazole compound per se or a light-catalyzed reaction of the dihydro benzimidazole compound under the action of a catalyst by using ultraviolet light or visible light, and the catalyst used in the preparation of the benzimidazole cationic compound and the hydrogen can be recycled.