Direct current（DC）flowing with the first winding current of current transformers（CT）causes a DC component in the magnetizing current. And it directly impacts the accuracy of measurement and power metering.This paper qualitatively analyses that errors of CTs increase with DC bias uses the equivalent circuit models and simulation of magnetizing current. Based on a full-wave Fourier algorithm

a suitable approximate solution of electromagnetic equation provides expressions for the ratio error and phase displacement. Simulation and case study are given to verify the above analysis results

and the computed results agree well with the simulated ones.The 3D error curves are quantitatively obtained to show that the ratio error and phase displacement increase with the DC bias

which provides scientific basis for measuring the design and specifications of the CT.