The transferring characteristics of current transformers（TA） are subject to possible changes due to DC magnetic bias caused by geomagnetic induced current（GIC）/high voltage direct current（HVDC）. The particularity of TA saturation affected by the DC magnetic bias will severely impact the transformer differential protection system. Based on the AC system simulation models considering DC magnetic bias， the effects caused by the variation of time-to-saturation of protective TAs are studied. Moreover， studies show that DC magnetic bias will lead to TA partial transient saturation. According to simulation of sympathetic inrush with DC magnetic bias， this paper analyzes the impacts on differential protection resulting from the TA partial transient saturation. The research demonstrates that DC magnetic bias may accelerate saturation of protective TAs. This is a challenge to the method of locating the time difference used for preventing maloperation due to TA saturation when external faults occurred. The research also shows that TA partial transient saturation caused by DC magnetic bias may contribute to maloperation of transformer differential protection.