The vortex-induced vibration experiment of a free-hanging segmented riser is carried out in a towing pool. The dynamic response of the segmented riser is studied by changing the speed of the trailer to drag the riser to generate different flow velocities. The riser strain response， dominant frequency， displacement response and other parameters are analyzed based on the modal analysis method， and the dynamic response law of typical pipe sections is discussed. Then the mechanism of vortex-induced vibration of free-hanging segmented riser is explored. The results indicate that the free-hanging riser undergoes different degrees of torsion under the influence of vortex-induced vibration. The greater the reduced velocity means the stronger the influence degree， and the effect of drag force on the vibration response of a free-hanging riser cannot be ignored in the cross-flow direction. The middle and lower positions of the riser show obvious modal competition under the effect of vortex-induced vibration. At low reduced velocities which are different from the conventional boundary conditions （hinged at both ends or fixed at both ends）， the dominant frequencies of the vortex-induced vibration of the riser are doubled in the two directions， which are consistents. The change trend of the displacement amplitude in the cross-flow direction of the riser is opposite to the in-line direction， and the modal conversion range lags in the in-line direction behind the cross-flow direction， resulting in modal conversion not occurring in the same reduced velocity range.