This application relates to an overload and unbalanced load detecting system for a railway and a detecting method. This system includes at least one steel rail. A rail web of each steel rail is provided with two sampling points at two sides between every two adjacent rail sleepers, respectively, and the two sampling points on one side are symmetrically disposed about the steel rail with respect to the two sampling points on the other side. A fiber-optic sensitive element used for continuously measuring a load when a train passes through the two sampling points is obliquely fixed at each sampling point, and two fiber-optic sensitive elements on the same side of each steel rail are disposed at an angle of 90° with each other.