A first optical network device groups a plurality of FlexO instance frames into one group, where each of the plurality of FlexO instance frames carries one OTU signal; then, performs multiplexing on the plurality of FlexO instance frames grouped into one group, to generate one first FlexO frame; next, performing scrambling and FEC processing on the first FlexO frame to generate one second FlexO frame and send it to a second optical network device. If a rate of the FlexO instance frame is 100 Gbps and two FlexO instance frames are grouped into one group, the 200G optical module can be used in the transmission method.