A sphere decoding method applied to a MIMO channel is provided. T signals transmitted via the MIMO channel are received. A first triangular matrix corresponding to a channel matrix is generated and mapped from the complex domain into the real domain to obtain a second triangular matrix. A first zero-forcing soft-output solution corresponding to a first estimation layer is found, and multiple preferred points P(1) are obtained. Multiple n-th zero-forcing soft-output solutions corresponding to an n-th estimation layer are obtained according to multiple preferred points P(n−1), and multiple preferred points P(n) are obtained according to PEDs of multiple n-th constellation points. Multiple 2T-th zero-forcing soft-output solutions are obtained according to the preferred points P(2T−1) and multiple preferred points P(2T) are obtained correspondingly. The preferred point P(2T) corresponding to the least PED is mapped from the real domain into the complex domain to generate an optimal solution of the T signals.