An optical signal transmission method includes mapping a first optical data unit frame to a first flexible tributary unit frame, where the first flexible tributary unit frame includes a plurality of payload blocks; mapping the first flexible tributary unit frame to a first optical payload unit frame, where the plurality of payload blocks are distributed in a payload area of the first optical payload unit frame; mapping the first optical payload unit frame to a second optical data unit frame, where a bit rate of the second optical data unit frame is greater than a bit rate of the first optical data unit frame; mapping the second optical data unit frame to a first optical transport unit frame; and sending the first optical transport unit frame.