Effective vibration control technology for stay cables is extremely critical to safe operations of cable-stayed bridges. This paper mainly foucs on a new vibration control technology based on an electromagnetic damper for stay cables. First, both mechanical and electrical performance of a rotary electromagnetic damper was tested. On this basis, a test platform consisting of an electromagnetic damper and a model cable was established in the laboratory to evaluate control performance of the cable, where different damper locations and transmission ratios between linear and rotation motion were investigated. The test results shows that the proposed vibration control system can greatly improve modal damping ratios in each of first five modes of the cable. The control performance of the cable is propotional to the installation height of the damper and transmission ratios. It it further demonstated that excellent control performance of the electromagnetic damper is mainly due to two aspects: one is that it can work as a linear viscous damper, and the other is it can realize negative stiffness control.