The aim of this paper is to numerically investigate the energy extraction performance of a flexible hydrofoil under the semi-activated mode. The effects of flexure amplitude (α), pitching amplitude (θ0) and reduced frequency (f∗) on power extraction were studied and compared with rigid hydrofoils. The evolution of effective angle of attack, vortex and pressure field are examined. The results show that averaged power coefficient (Cp¯) and efficiency (η) of the flexible hydrofoil are better than that of rigid hydrofoils. The increase of α can obviously improve the zones of high η at 0.15 < f∗<0.25 and 50° < θ0 < 80°. Especially, increasing α could obviously improve Cp¯ and η at a low θ0. Further, at a high θ0, the factors causing decline of power extraction are the separation of leading-edge vortex at low frequency and the shifting of pressure center caused by the decrease of pitching angular rate at high frequency. Compared with the rigid hydrofoil, the flexible hydrofoil can increase the pressure difference between the upper and lower surfaces of hydrofoils, thus lift is improved, which is beneficial to the improvement of energy extraction performance.