Abstract: A series of tests on vortex-induced vibration (VIV) suppression for an inclined flexible straked cylinder with a high aspect ratio (L/D=350), low mass ratio (m^*=1.90) and large inclination angle (a=45 °), were conducted in a towing tank. The effect of helical strakes with a pitch of 17.5D and a height of 0.25D on the VIV reduction of an inclined cylinder was investigated and discussed. The dynamic response features of an inclined straked cylinder undergoing VIV were studied. The uniform flow was generated by towing the cylinder model in still water along the tank from 0.05 m/s to 1.0 m/s with an interval of 0.05 m/s, the corresponding Reynolds number ranged from 800 to 16000. The strain signals in cross-flow (CF) and in-line (IL) directions were measured using strain gauges. The response displacement was obtained by a modal analysis approach. The experimental results indicated that the strain and displacement suppression efficiency of the inclined cylinder attached with helical strakes are not as well as that of a vertical straked cylinder. Moreover, the inclined flexible cylinder fitted with helical strakes exhibits regular CF and IL multi-modal VIV. The independence principle (IP) seems to be an unreasonable hypothesis for predicting VIV of an inclined flexible straked cylinder with large inclination angle.