This application provides a data transmission method in an optical network and an optical network device. The method includes: obtaining, by a first device, first synchronization information from a first service data stream, and determining a to-be-transmitted service data stream; generating second synchronization information based on the first synchronization information; mapping the second synchronization information and the to-be-transmitted service data stream to an optical carrier container; and sending the optical carrier container. A second device receives the optical carrier container; obtains a second service data stream and third synchronization information from the optical carrier container through demapping; generates fourth synchronization information based on the third synchronization information; and inserts the fourth synchronization information into the second service data stream, to obtain a third service data stream. In this way, time synchronization precision is improved.