Aiming at fault point locating problem of the line protection multiplexing optical channels, the digital multiplexing device and protection device detect the channel fault flags by the validity of physical layer encoding, generate a status stamp at reserved field by expanding communication message, and use heartbeat frame mechanism of digital multiplexing device to solve the problem of exchanging status stamp when error occurs in channel. The fault point location of the channel can be detected on the protection device directly. Fault self-diagnosis mechanism is implemented for multiplexing optical channels.