Embodiments of this application disclose a data transmission method, an apparatus, a device, a system, and a readable storage medium. The data transmission method includes: A first module obtains first data, where the first data is data that is encoded by using a first FEC code type and on which first processing is performed, and the first processing includes interleaving processing. The first module encodes the first data based on a second FEC code type, to obtain a plurality of second codewords. The first module performs second processing on the plurality of second codewords, to obtain second data. The first module transmits the second data.