The complex response of the ocean thermal energy conversion platforms in the marine environment makes the structural safety design of cold seawater intake pipes suspended beneath the platform a challenge. Recent research has shown that the in-plane motion of the platform （heave oscillation） triggers out-of-plane vortex-induced vibration （VIV） in the riser connected to the plat? form and the complex vortex vibration response can cause rapid accumulation of fatigue damage to the riser， resulting in structural damage. In this paper， we focus on the VIV response of a free-hanging riser under multi-degree-of-freedom motion， which has rare? ly been reported in previous studies. A pool model test to measure the VIV strain information of the riser using fiber-optic grating strain gauges is carried out. After analysis of the experimental results， it can be found that： the maximum oscillation velocity at a large KC number is the main parameter affecting the dominant frequency of the out-of-plane vortex vibration response； the domi? nant frequency of vibration at a small KC number is twice frequency of the motion of the top platform. By comparing the experimen? tal and numerical results of the free-hanging riser under the three degrees of freedom motion of the platform， it is found that the in? fluence of the VIV on the overall dynamic response of the free-hanging riser is not negligible in this case. These results can provide a reference for further research on the VIV of the free-hanging riser taking into account the influence of the platform motion.