A coding method for data communication is provided, and may be applied to a plurality of scenarios such as a metro network, a backbone network, and a data center interconnection. The method includes: forming a first codeword, 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 source codeword is a codeword formed before the first codeword, both n and m are positive integers, and n>m; and sending the n to-be-transmitted bits in the first codeword. The bit in the first codeword is protected by a plurality of codewords generated at different moments, and a coding gain effect is better. In addition, the bit in the codeword is protected by different quantities of codewords.