DC bias may affect the transferring characteristics of the current transformer，threatening the operation of differential protection.Compared with the DC bias caused by high voltage direct current（HVDC）transmission，the bias magnetic flux resulting from geomagnetically induced current（GIC）is more complex，especially when considering its effects on current transformer transferring characteristics.The differences between GIC bias and DC bias are analyzed in detail.According to the equivalent circuit of current transformer，the mechanism of current transformer saturation due to GIC is discussed.Then based on the characteristics of GIC，simulation models are developed in PSCAD/EMTDC，where low frequency sinusoidal currents are adopted to emulate GIC.Simulations are conducted to analyze current transformer transient saturation and partial transient saturation，the results of which demonstrate that effects of GIC may result in mal-operation of transformer differential protection，and should be given prompt attention.