The current transformer (CT) used for protection has a great effect on operation characteristics for protective relaying. Its transfer characteristics could be impacted by DC bias and its excitation current and magnetic flux is derived by T-shaped equivalent circuit. Then the analysis of simulation on different DC offset ratio by means of PSCAD/EMTDC software. The simulation result shows that it will cause CT’s magnetization curve in third quadrant decrease and its secondary current distortion meanwhile the harmonic of secondary current increase and when the DC offset coefficient increase to a certain value the second harmonic will be greater than the third harmonic, but the fundamental is reduced accordingly. And the transfer characteristic of protective CT could be analyzed by simulating DC bias with the designed AC superposed DC experiment circuit. The result of experiment shows that the CT can not transfer the primary current accurately on DC bias condition, the secondary effective current decreases on DC bias and the fundamental decreases and others harmonic increases, and the impact to the phase error more serious than current error. So, the protective CT’s transfer characteristic is changed and it’s a serious threat to protection used for fundamental algorithm and harmonic restraint scheme which can reduce the reliability of relay protection.