The invention relates to a method for forming differential current based on the current slow attenuation of a differential current zero crossing identification CT (computed tomography), which comprises the following steps: 1) calculating the three-point integral value of the present point differential current according to the formula S(n)=x(n) + x(n-1) + x(n-2) and comparing the three-point integral value with the three-point integral value S(n-1) of the former point differential current, wherein the present point is judged to zero crossing point if S(n)*S(n-1) is less than 0; and 2) the internal faults are judged if more than two zero crossing points occur in the time T, otherwise, the differential protection is locked. The incorrect false operation of the differential protection in the course of the differential current resulted from the current flow attenuation of the CT is realized when the external faults are eliminated.