1. A multiple-input multiple-output (MIMO) detection method for a receiver in a MIMO system using N-quadrature amplitude modulation (N-QAM), the method programming a processor circuit to implement the method, the method 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; selecting a second candidate symbol vector set conformed to a constraint in the first search space when the level of the first search space is not the lowest level of the multilevel structure, wherein the constraint corresponds to the level of the first search space, wherein the constraint comprises a Euclidean distance of an error vector between each symbol vector of the first candidate symbol vector set passed through channels and a received signal is smaller than one of the plurality of search radiuses; generating a second search space according to the second candidate symbol vector set, wherein the level of the second search space is lower than the level of the first search space; 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, wherein generating the detection signal more specifically comprises performing one of the following operations: (1) calculating Euclidean distances of a plurality of error vectors, or (2) using a maximum likelihood algorithm in the search space for selecting a symbol vector.