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With the worldwide realization and application of High Voltage Direct Current (HVDC), the additional effects of HVDC transmission technology on power system have caused extensive concern. The DC magnetic bias caused by HVDC may exacerbate the saturation degree of the current transformer (CT) during the external fault, which probably leads to the mal-operation of the transformer differential protection. In this paper, the state of CT transient saturation is divided into two kinds. Saturated characteristic and error analysis are presented in detail. Afterwards, two cases of CT saturation with and without the DC bias current have been compared in order to verify the effect of the biased flux on the time-to-saturation of the CT. Furthermore, based on the fact that time-difference method fails to block the external fault under the acceleration of CT saturation, this paper proposes an improved protective scheme to re-partition the ratiorestrained characteristic and sets a time-delay action region to avoid the mal-operation of the differential protection. What is more, CT saturation will affect the performance of the protective relay and threaten the security of power system, to which should be paid adequate attention.