A method and device for indicating an uncorrectable data block. The method includes: if a forward error correction decoding fails, setting synchronization character of at least one of the corresponding data blocks to a first character; and performing line decoding on the data block with the set first character, and outputting decoded data. With the invention, indicating the uncorrectable data block Simple and effective to a line decoding module can be implemented in case of a failure of FEC decoding.