The negative stiffness behavior of a damper can improve its performance for cable vibration control. To explore the performance enhancement effect of the negative stiffness（NS）device on the viscous damper（VD）in mitigating cable vibrations，the combined effect of the VD and NS for cable vibration control is investigated. The characteristic equation of the cable-VD-NS system is established based on the complex modal method. On the basis，the asymptotic solution of the cable modal damping ratio is derived. The effect of the NS coefficient and the NS location on the modal damping ratio and vibration frequency of the cable is evaluated. Based on the constrained static output LQR method，the design parameters of the VD for a given NS coefficient for suppression cable multi-mode vibrations are obtained. Furthermore，the effect of the NS coefficient and the NS location on the VD performance in mitigating cable multi-mode vibrations is investigated. The results show that increasing the NS coefficient or the NS location can improve the control performance of the VD and reduce the optimal damping coefficient of the VD，including the singlemode and multi-mode vibrations of the cable. The performance enhancement of the NS on the VD is mainly attributed to the increment of the NS coefficient or the NS location can amplify the displacement of the VD，which can improve the energy dissipation capacity of the VD.