This paper investigates the influence of dampers on cable vibration control. The mode shape of the cable changes after the installation of dampers, which changes the dynamic characteristics of the system. Thus, an Improved Separation of Variable Method is proposed to model the vibration behavior of cable–damper systems, including the amplitude and phase of cable vibration. The analysis based on this method reveals that the damper affects the entire cable vibration through the phase difference, which is closely related to the amplitude of vibration. The nonlinear ordinary differential equations obtained in the previous step are solved, and the theoretical solutions of the amplitude and phase are obtained. To validate the theoretical solution, a free vibration test of cable of 19.56 m is conducted. Finally, a parametric analysis of the cable–damper model was conducted based on the verified theoretical framework. The results indicate that the amplitude decay rate of the cable’s vibration initially increases and then decreases with an increasing damper coefficient, and the frequency change depends on the vibration mode and damper position. Furthermore, after the damper is installed, the motion of the cable exhibits a traveling wave effect, which results in a damper clamping effect on the cable due to the phase difference of the traveling wave. The application of parallel negative stiffness can reduce the clamping effect of lower modes of the system; however, its effectiveness is not significant for higher modes.