Abstract: In order to study the variation of the horizontal additional tension of the large-span CFRP cable under the vertical seismic action with the change of the span and sag-span ratio, aforced vibration equation for a suspended cable is derived based on the linear free vibration theory. Further more, a vibration mode parameter α_n is obtained for the approximate analysis of the additional tension. The calculation of the parameter α_n which is used to research the variation of the horizontal additional tension is based on the static parameter, calculated by limit design principle, of the CFRP suspended cable for two kinds of suspension bridge. The result demonstrateed that the change of the span of CFRP cable had hardly influence on the parameter α_n and a small change of the sag-span ratio obviously affects α_n, The second vibration mode was the main mode contributing to additional tension of suspended cable for the situation without the backstay. In addition, with the increase of the sag, the third mode should be considered also for its contribution to the additional tension. When the suspended cable had the backstay, the first two modes contributed the majority of the additional tension. However the contributions were affected by the sag ratios.