A data transmission method includes receiving, by an optical line terminal (OLT) from an optical network unit (ONU), uplink burst data that includes a synchronization data block and a payload, where the synchronization data block includes first synchronization data, wherein the first synchronization data includes a first preamble and an ONU identifier, and a first bandwidth occupied by the first frequency distribution of the first synchronization data is narrower than a second bandwidth occupied by the second frequency distribution of the payload, and obtaining, by the OLT from the first synchronization data, the ONU identifier.