Embodiments of the present invention provide a forward error correction codeword synchronization method, device, and system. The method is: sending, by a central office device, synchronization information of an FEC codeword to a terminal device by using a management channel, where the information includes information about an agreed location of an FEC codeword, and the information about the agreed location of the first FEC codeword indicates a location that is of the first FEC codeword and is corresponding to data at an agreed location of an agreed time-frequency resource block; and receiving, by the terminal device, the synchronization information that is of the FEC codeword and is sent by the central office device, and adjusting a status parameter of an encoder or a decoder according to the information, so as to complete codeword synchronization. The embodiments of the present invention are used for FEC codeword synchronization.