Abstract: Long-span cables are susceptible to vibration with large amplitude under wind/rain loadings or support excitations. To overcome these problems, a theoretic model based on the Hamilton principle for the cable-damper system is formulated by accounting for the cable inclination and sag effect. The discrete Magneto-Rheological (MR) dampers are installed on the 330m-long cable in a stay-cable bridge. The dynamic response performances of a cable with MR dampers are explored by using Spencer photometric model. Based on the direction of both displacement and velocity, the semi-active control strategy of cable vibration is proposed. The primary measure of damper performance is the Root Mean Square (RMS) considered cable deflection integrated along the length of the cable and the whole time period. The comparison study among the semi-active under the strategy and the optimal passive MR damper has shown the advantages of semi-active dynamic adjust control when MR dampers marker is within the effective region. The strategy is robust and quite easy to be applied for fewer necessary sensors. The dynamic range of MR dampers is sufficiently wide. The resonant frequencies of the cable installed MR dampers have a little change. It implies that MR dampers under semi-active control strategy can significantly reduce long cable vibration.