Abstract: By comparing the energy consumption between a linear viscous damper and a friction damper, it can been found that the maximum modal damping ratio of a cable with the friction damper is larger than that for an optimal passive linear viscous damper. This paper proposed a semiactive control strategy of stay cables with a variable friction damper force. The study shows that the friction force level has to be adjusted in proportion to the cable displacement amplitude at the damper position, and keeps cable’s vibration to be always in the optimal state when the friction force and the damper displacement are corresponding optimal value. Consequently, the maximum modal damping ratio can be obtained. The semiactive control algorithm is independent of vibration frequency and suitable for the vibration control of multi-modal coupling in the cable. The potential of using semiactive control algorithm to control stay cable vibration has been demonstrated by numerical simulation with several control designs. Either free vibration or harmonic forced vibration of a cable can be effectively reduced.