The invention relates to a desulfurizing bactericide. The desulfurizing bactericide comprises the following components by weight percent: 20-30% of dodecyl dimethyl benzyl ammonium chloride, 5-10% of poly-dimethyl diallyl ammonium chloride, 1% of an emulsifying agent OP, 20-30% of polymeric ferric sulfate, and the balance of water. The preparation method comprises the following steps: adding dodecyl dimethyl benzyl ammonium chloride, poly-dimethyl diallyl ammonium chloride and the 1% of emulsifying agent OP in the water for completely dissolving, adding polymeric ferric sulfate, uniformly stirring to obtain the desulfurizing bactericide. In the early stage of the application, the dashed dosing concentration is 120-150mg/L, after the concentration of hydrogen sulfide has a downward tendency, the addition concentration of the product is adjusted to 60-80mg/L; through the adoption of the desulfurizing bactericide, the sterilizing rate of the sulfate reducing bacteria is not less than 99%, the hydrogen sulfide content is not more than 10mg/m<3>; the sulfured hydrogen poisoning accident is prevented and avoided, and the damage to the equipment from hydrogen sulfide is reduced, the long-term operation of the system is guaranteed, the stability of the purified water quality is well guaranteed.