The residual flux is an important factor which affects the transfer characteristic of current transformer.The measuring residual flux is of important significance to the application of current transformer.In order to effectively measure residual flux and its coefficient,a new method to measure residual flux with alternating current voltage source is proposed.In the process of measuring residual flux,the current transformer is deeply magnetized by applying alternating current.By recording the induction voltage of current transformer and drawing the curve of core flux,the residual flux and its coefficient are calculated.The experimental results show that the measured residual flux and its coefficient is consistent with theoretical analysis.Besides,the proposed method can accurately measure residual flux of current transformer.