This paper analyzes the difference of transient characteristics of magnetic current transformer and electronic current transformer, and proposes to adopt the method of difference virtual brake CT saturated open to solve the problem of saturation mis open of line optical differential protection when different CTs are used mixedly. This method can effectively solve saturation mis open caused by inconsistent transient transfer characteristics. In addition, secondary time constant of electronic transformer has great difference from that of conventional current transformer, and it results in the current trailing of electronic transformer after fault excision, which can cause line protection still has calculation of differential flow after switch off. And the inconsistent secondary current attenuation time constants of CT at each side of protection can lead to misoperation of differential protection in cutting off the external fault. This paper puts forward the full-circle differential Fourier to calculate the current of jumped phase of protection, in order to avoid mal-start failure caused by not withdrawing jump command of line protection in time and the differential maloperation owing to cutting off external fault.