Demagnetization is required compulsory verification in testing current transformers. This article analyzes the respective merits and weaknesses of open circuit method and closed circuit method, and eventually proposes the open circuit voltage method of demagnetization. According to the law of electromagnetic induction, this article provides the technique to test the excitation admittance of the current transformer while testing current transformers in open circuit with voltage method, as well as researchs on how to select the test line to check admittance and demagnetization voltage. Finally the article reaches the conclusion that while using voltage method to demagnetize, magnetic conductivity is the highest when admittance is the lowest. This method both achieves the purpose of demagnetization and assures the safety of demagnetization experimentations.