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Due to the different ratings of the current transformers (CT) located on different sides of power transformer, only the CT of low ratings will saturate when the power transformer experiences a heavy through fault, leading to false differential current. Such type of external fault can be identified from the internal one if the differential protection is equipped with the percentage restraint characteristic together with the method using operation time difference between pickup element and differential protection. However, the differential protection will be wrongly blocked as well if a cross-country fault occurs. According to the investigations in this paper, in the event of an external fault accompanied by the CT saturation, the variation of most samples of the secondary current of the saturated CT is inversely proportional to the variation of the differential current. Comparatively, this law cannot be followed on the occasion of an internal fault. In this case, the locus of the variation of the saturated secondary current with the differential current can be used to dynamically discriminate if an external fault develops to an internal fault. With the method proposed in this paper, the ability of the differential protection immune to cross-country fault can be improved further. The effectiveness of the proposed method is verified with the simulation tests.