Vibration responses of 500 m scale cable-stayed cables of Sutong Bridge show that：Ultra-long cables may have severe high-order vortex-induced vibrations，and the mode orders of rain-wind-induced vibrations are much higher than that of traditional medium or short cables. The traditional parameter optimization schemes of viscous dampers for controlling the first several modes of stay cables cannot meet the requirements of high-order vibration control of ultra-long cables. In this paper，the additional modal damping ratios and the optimal damping coefficients of the cable-viscous damper system are derived. Moreover，modal damping ratios are taken as the control index. The parameter optimization scheme of viscous dampers suitable for controlling the multi-modal vibration of ultra-long cables is studied，and compared with the traditional schemes. The results show that there is a great difference between the optimal damping coefficients of the adjacent lower modes，which leads to a sharp decline of the control effect with the increase of the mode order when the damper parameters are designed according to the first several modes. With the increase of mode order，the difference between the optimal damping coefficients of adjacent modes will decrease. If the control of the first and second modes is not considered and the parameter optimization of the damper starts from the third mode（ζ3> ζmin），the modal range of control can be greatly expanded.