1. A multiple-input multiple-output (MIMO) detection method for a receiver in a MIMO system using N-quadrature amplitude modulation (N-QAM), comprising: generating a plurality of symbol vector sets and a plurality of search radiuses, each symbol vector set and search radius correspond to one level of a multilevel structure of N-QAM constellation; selecting a first candidate symbol vector set at a highest level of the multilevel structure; generating a first search space at a lower level of the multilevel structure according to the first candidate symbol vector set; confirming which level the first search space corresponds to; and generating a detection signal according to the first search space when the first search space is the lowest level of the multilevel structure.