The invention discloses a method for enhancing degradation of organic arsenic in waste water of livestock breeding into pentavalent arsenic by utilizing weak potential. The method is characterized in that under the condition that a carbon source is exhausted, weak potential is added into an anaerobic treatment system, additional electron acceptors are provided for waste water of the organic arsenic, and the speed for degrading and converting the organic arsenic into the pentavalent arsenic by anaerobic microorganisms is improved. The treatment method has the advantages that the degradation enhancing effect to the organic arsenic is good, fast conversion from the organic arsenic into the pentavalent arsenic is realized, and the difficulties of high solubility, difficult removal and slow degradation of the organic arsenic in the waste water are solved.