The present invention relates to a maximum likelihood detector and a maximum likelihood detection method. The maximum likelihood detector comprises a searching value selection circuit and a maximum likelihood detection circuit. The maximum likelihood detection circuit is configured to execute the following at least steps: selecting K first layers of candidate signal values according to the receiving signals and derivation signals, channel estimation signals or derivation signals thereof and the first layer of searching values; calculating K second layers of candidate signal values according to the K first layers of the candidate signal values; determining whether P second layers of the supplemented candidate signal values are added or not according to the K second layers of the candidate signal values, and generating a determination result; if the determination result is yes, calculating a log-likelihood ratio according to the K first layers of the candidate signal values, the K second layers of the candidate signal values, the P first layers of the candidate signal values and the P second layers of the candidate signal values; and if the determination result is no, calculating the log-likelihood ratio according to the K first layers of the candidate signal values and the K second layers of the candidate signal values.