Embodiments of the present invention relate to the field of communications, and provide a decoding method and a decoder, which are used to reduce decoding complexity. The method includes: receiving a to-be-decoded signal; performing region decision on the to-be-decoded signal according to a region decision rule formed by S region decision formulas, to acquire a region decision result; acquiring N constellation points according to the decision result, where the N constellation points are separately constellation points that are in the N subsets and that are closest to the to-be-decoded signal; acquiring N non-encoded bits corresponding to the N constellation points, and branch metrics between the to-be-decoded signal and the N constellation points; and performing Viterbi decoding based on the branch metrics and the N non-encoded bits, and outputting a decoding result corresponding to the to-be-decoded signal. The present invention is applicable to a signal decoding scenario.