To solve the reliability assessment problem of protection channel caused by elements multiplexing. The paper builds a weighted-undirected graph of fiber protection channels, whose nodes measurement is SDH reliability and reliability of fiber optical as edge weights. It starts from the information provided by the actual grid, encodes the data according to the most close to the actual data format, and develops an algorithm for channel and service reliability assessment based on logical "or" and "exclusive or". The result is verified in a 18-node and 19-channel network in central China Grid, showing that the proposed encoding method and model have good practicability and operability, which can offer grid reference to network optimization, scientific channels arrangement and the maintenance of the protection channel.