A new channel monitoring method for the fiber-optic longitudinal differential protection is put forward which consists of the following steps， viz. correspondence between the differential protections at the two ends of the line in the designed message frame format; the communication channel delay in calculating and storing each message frame; judgment of the stability of channel delay according to the criterion inequality. If the communication channel delay satisfies the criterion inequality， it is concluded that the channel is unstable and the closedown protection. The method bypasses the risk in routing switching by monitoring the channel delay jump， and by taking into full consideration the guarantee of the reliability of differential protection in the process of communication channel routing switching， the maloperation accident of protections is eliminated. The criterion focuses on the time of channel delay jump rather than the channel delay itself and is adaptable to channels with diverse delays. The application results verify that the proposed method has high sensitivity and reliability.