Embodiments of the present disclosure provide a data synchronization method and system, and an optical network unit. An ONU receives synchronization data that is transmitted by a first OLT through a GEM port corresponding to a predetermined GEM port ID or a logical link corresponding to a predetermined LLID, and stores the synchronization data. When a fault occurs on the first OLT or on a backbone optical fiber connected to the first OLT, the ONU transmits the synchronization data to a second OLT so that the second OLT recovers services according to the synchronization data.