Method to prepare domidin (4-isothiazol-3-one, 3-hydroxyl-isothiazole). Blowing chlorine gas through 3,3'-dithio-dipropionamide in acetic acid solution, than treating with alkali. The intermediate 3.3'-dithio-dipropionamide is made by reaction in water solution of propionamide and sodium sulfide in presence of sodium ficarfonate, and then treat the resultant with sodium sulfite sclution. Pcisonous hydrogen sulfide gas is not required, all reactants are inexpensive and product yield is high. This preparation procedure is simple and easily commercialized. Domidin is a sterilizer and germicide.