Abstract: Vortex-induced vibrations (VIV) may cause serious fatigue damage on flexible cylinder structures, such as marine risers, mooring lines of floating platform and free spanning pipelines. Nowadays, in ocean engineering application, the hydrodynamic coefficients, used to predict the VIV of flexible cylinders, are mainly acquired from the forced oscillations of rigid cylinders in cross-flow, which may account for the unexpected errors. Taking the coupling between cross-flow (CF) and in-line (IL) vibrations into account, a fluid force model is proposed in this paper. Lift, impulse drag and added mass coefficients are obtained by the finite element method and least square method. A series of experimental tests of a flexible cylinder with a mass ratio of 1.82 and an aspect ratio of 195.5 are conducted in a towing tank to investigate the hydrodynamic coefficients on the flexible cylinder undergoing VIV. Comparing to the hydrodynamic characteristics of rigid cylinders, those of a flexible cylinder are further studied. Under the first-order mode, the hydrodynamic coefficients of a flexible cylinder share the same variation with those of rigid cylinders. A remarkable growth on lift and impulse drag coefficients is observed under the second-order mode. The added mass coefficients are strongly related to the displacement as the frequency is low. The large displacement region is the energy dissipation region with low frequency. While, the large displacement region may turn into the energy input region with high frequency.