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A new converter consisting of an elastically mounted circular cylinder and a free-to-rate pentagram impeller is proposed to harness hydrokinetic energy from water currents. The vibration energy of the cylinder and the rotation energy of the impeller are harvested simultaneously. The two-way fluid-structure interaction simulations are employed to investigate the vibration and rotation response of the converter. The simulated Reynolds number range is 14,000 < Re < 80,000, falling in the TrSL2 (transition of shear layer 2) and TrSL3 regimes. The results indicate that the vibration amplitude of the converter increases with increasing of the flow velocity, and the growth becomes faster at high reduced velocity (Ur). The presence and rotation of the impeller contribute to the hydrodynamic instability of the system at Ur ≥ 9, where the rotational direction is constantly changing. The hydrodynamic instability brings about a wider flow wake and unstable vortex shedding, resulting in the enhancement of vibration and the increase of power. For such a rotation symmetric structure, increasing the torsional friction does not play a significant role in suppressing the hydrodynamic instability. The estimated power generating capacity is sensitive to flow velocity and additional damping ratio. The average power efficiency is about 22.6%, and the power density for a 5D × 5D staggered configuration with ζharn of 0.031374 can reach 885.53 W/m3.