With the span of cable-stayed bridge increasing, the staycable becomes longer to obtain the close fundamental frequencies of bridgeand cable, and to increase the probability of coupled vibration betweencable and girder. Generally, the dynamic characteristics of long cable aredetermined by the interactions among pylon, cable and girder. One of themost effective vibration mitigation measures is to use mechanical damper forlong stay cables of large span cable-stayed bridges. In the conventionaldesign theory for cable damper, it is assumed that the cable's ends arefixed and one end of the damper is connected with cable and the other end isfixed to girder. However, when the span of cable-stayed bridge becomes largeand the structure becomes more flexible, the effect of girder vibration onthe cable damper performance should be considered. Furthermore, the dampingratio in theoretical analysis is always smaller than in the full-scale cablevibration mitigation tests on actual cable-stayed bridges. Therefore, therelation between girder vibration and the efficiency of cable damper isinvestigated in this paper. A simplified theoretical model with cable,Kelvin damper and girder is analyzed by complex mode method. Model tests arealso carried out to study the influence of girder vibration on the modaldamping ratio of cable. Both theoretical and experimental results show thatthe girder vibration should be taken into account in cable damper design forlong span cable, especially for the coupling vibration with cable andgirder.