The error code ratio and the coherence of bi-directional channel time-delay of fiber channel are two important factors influencing performances of longitudinal differential protection. A method for recognizing the coherence of bi-directional channel time-delay by using difference between voltage phase angles calculated with line lumped-parameter model is proposed. In circumstance of single channel， two transmission rules， i.e. short frame fit for pilot protection and long frame fit for longitudinal differential protection， are combined， so that the seamless handover between the two kinds of protections can be automatically implemented according to the channels circumstance. The tests show that the seamless handover scheme can significantly reduce the exiting time of main protection， thus improve the reliability of protection.