Longitudinal viscous dampers are usually installed between tower and girder to control the girder＇s longi- tudinal vibration，which is the dominated mode to the dynamic reactions of a floating-system cable-stayed bridge， and the rationale damper parameter has to be determined through complicated FEM analysis． In the light of the dominated vibration mode contribution in floating system，a 3 DOFs simplified dynamic model considering the su- pplemental viscous damper that improves the mechanical behavior of 2 DOFs model was proposed，and the formula- tions for calculating the dynamic responses subjected to pulse dominated near-fault ground motions were deduced． Finally，with an actual bridge project as an example，through comparing the difference between the various simpli- fied model and the whole bridge model，it was proved that the 3 DOFs simplified dynamic model can not only effec- tively calculate the seismic dynamic response of cable-stayed bridge under the action of impulsive earthquake，but also provide a more accurate relative displacement difference and velocity difference between the tower and girder． It is conducive to a more reasonable and accurate design damper parameters，and the rationality of the model is verified．