1. A method for searching a solution point of maximum-likelihood (ML) detection, wherein the solution point locates at a symbol constellation, the method comprising: determining a central point and a norm by a zero-forcing detection method, wherein the solution point of ML detection locates inside a sphere that utilizes the central point to be a center and the norm to be a radius; determining a searching range according to the central point and the norm; determining at least one qualified solution point according to the searching range; and determining the solution point of ML detection from the at least one qualified solution point.