This application provides an encoding method, comprising: encoding first data to generate a first encoded sequence; encoding second data to generate a second encoded sequence, where a quantity of bits comprised in the first data is greater than a quantity of bits comprised in the second data, and a quantity of bits comprised in the first encoded sequence is equal to a quantity of bits comprised in the second encoded sequence; generating a symbol sequence by performing a PAM-4 modulation on an input signal, the first encoded sequence corresponds to a high bit of the input signal, and the second encoded sequence corresponds to a low bit of the input signal; and sending the symbol sequence. The method helps reduce a quantity of redundant bits, helps improve a code rate, and can reduce redundant information carried in a modulated symbol sequence.