In this paper, the calibration and workability of magnetoelastic (ME or EM) stress sensors for large steel cables used in Qiangjiang 4th bridge in China are discussed. As an engineering application of magnetoelasticity, EM sensors make non-contact stress monitoring possible for steel hangers and post-tensioned cables on suspension and cable-stayed bridges, and other ferromagnetic structures. By quantifying the correlation of tension with magnetic properties represented by the relative permeability of the steel structure itself, the EM sensor inspects the loading level of the steel structure. The tension dependence of the relative permeability and the temperature influence was calibrated. The results revealed that the magnetoelasticity of the multi-wire hangers is consistent with one another, while the post-tensioned cables are similar to single wires. Cable stress monitoring on Qiangjiang (Qj) 4th bridge demonstrated the reliability of the EM sensors in safety evaluation of bridge.