A data processing method and a device, the method including determining, by an optical network unit, in a received data stream, after the optical network unit enters a pre-synchronization state, a position of a forward error correction (FEC) codeword boundary in the data stream based on a position of a physical synchronization sequence (Psync) field, performing an FEC codeword decoding check, where the Psync field is a field that matches a preset Psync, in the data stream, and entering, by the optical network unit, a synchronization state in response to an FEC codeword among N consecutive FEC codewords in the data stream passing the decoding check, where N is an integer greater than or equal to 1.