A method and an apparatus for generating soft-decision information in a multiple antenna system are provided. The method includes determining Q candidate symbol vectors for a first transmission symbol of a received signal vector by performing parallel detection on a received signal vector and a channel matrix, determining a candidate symbol vector having a shortest Euclidean distance to the received signal vector from among the Q candidate symbol vector, as an approximate Maximum Likelihood (ML) symbol vector, determining (Q−1) candidate symbol vectors for each of the remaining transmission symbols of the received signal vector by performing partial parallel detection on the received signal vector and the channel matrix using the approximate ML symbol vector, and calculating Log Likelihood Ratios (LLRs) of bits of the first transmission symbol using the candidate symbol vectors, wherein Q represents a modulation order.