Embodiments of the present invention provide a decoding method and apparatus. The method includes: acquiring a demodulation signal; acquiring a first decoding signal, where the first decoding signal is a signal fed back after the ith M-ary differential decoding processing is performed on the demodulation signal, and i is an integer greater than or equal to 0; and performing M-ary differential decoding processing on the demodulation signal according to the first decoding signal, to obtain a second decoding signal. The first decoding signal that is fed back is added and input to a decoder.