Abstract: Cables of large-span and high-rise structures such as bridges are prone to multi-mode vibrations under environmental excitations such as wind and rain. The installation of dampers on the cables is a common countermeasure for vibration reduction. With the increase of cable length, multi-mode damping of a cable provided by the damper installed near the cable end is hard to meet the design requirement. This study proposes a friction damper with negative stiffness (NSFD) for cable vibration control and damping enhancement, which achieves negative stiffness and provides the normal force to the friction surface through a pre-compressed spring. Linearized negative stiffness coefficient of the NSFD is first derived, and the damping effect of NSFD on the cable is analyzed by energy equivalence method. The reduction in structural responses of the cable is further analyzed by a numerical model of the cable-NSFD system. Numerical simulations of a cable of a real bridge show that, when the NSFD has a negative stiffness coefficient of −150 kN/m, the maximum damping is increased by at least 26% for multiple modes as compared to a traditional friction damper. This study has theoretically shown the performance and feasibility of the proposed NSFD, and experimental validation will be conducted in future.