The accuracy of current transformer (CT) is important guarantee of power system protection, monitoring and measurement. A new method based on live working technique for CT accuracy evaluation is proposed in this paper. Through constructing standard part, which is on-line insert into power lines, and comparing the data of standard part with the data of the measured part, the system can calculate the ratio error and phase displacement of the measured CT. The results of on-line test have shown that the proposed method is feasible and effective, and the accuracy of proposed system achieves 0.1 grades.