Abstract: A vortex-induced in-line vibration model is formulated for the calculation of nonlinear dynamic response of submerged floating tunnel (SFT) tethers due to wave and current effects. The effect of tunnel surge motion on the dynamic response of a tether is investigated by considering the forcing and parametric excitations. Galerkin method and numerical integration were used in the study. Numerical relationship between the excitation frequency and the maximum amplitude of vortex-induced in-line vibration was obtained. The calculated results show that the amplitude of vortex-induced in-line vibration increased when considering the effects of forcing and parametric excitations and the amplitude approached its maximum when the excitation frequency approached the first-order natural frequency of the tether.