Analysis results of line protection mal-operation case show that, if a current transformer is in slight transient saturation when external fault occurs, zero-sequence differential protection is easy to malfunction because of the saturation criterion failure due to unobvious current waveform distortion. To solve this problem, transient saturation characteristics of three-phase current transformers are analyzed, by which the relationship between transient saturation time and fault angle and fault current is obtained, and minimum saturation time and minimum fully offset fault current that makes the current transformer saturated are calculated by iterative method. On this basis, improved zero-sequence differential protection starting element is constructed by using double-terminal zero-sequence current as the recognition object to identify the saturation state of current transformers. Simulated by all kinds of faults under residual magnetism and no residual magnetism conditions of the current transformer, the results show that the starting element can greatly improve the accuracy of zero-sequence differential protection by which its feasibility and effectiveness are proved. This work is supported by Science and Technology Project of State Grid Fujian Electric Power Company Limited (No. 52130416001U).