A coding method and apparatus for data communication are provided, and may be applied to a plurality of scenarios such as a metro network, a backbone network, and a data center interconnection. As an example method, a first codeword is formed, where the first codeword includes n image bits and n to-be-transmitted bits. The n image bits are selected from to-be-transmitted bits in m source codewords. The m source codewords are formed before the first codeword. Both n and m are positive integers, and n>m; The n to-be-transmitted bits in the first codeword are sent. The bit in the first codeword can be protected by a plurality of codewords generated at different moments. In addition, the bit in the codeword can be protected by different quantities of codewords.