The semi-passive flapping hydrofoil is a promising device for harvesting tidal stream energy in shallow waters with significant practical advantages compared to traditional rotor turbines. The energy-harvesting performance of a coupled-pitching hydrofoil under the semi-passive mode was studied experimentally and numerically. In the water channel test, a magnetic damper and an electric generator was employed to provide the constant and varying loads, respectively. The hydrofoil model performed better in energy-harvesting for the reduced frequency of 0.10 and the activated pitching amplitude of 70–90°. The two-dimensional and three-dimensional numerical models were established and the dual-sliding-overlapped-mesh technology was used to realize the complicated coupled-pitching motions and to predict the fluid-structure interaction of the hydrofoil. The vortex structures and corresponding pressure distribution characteristics over the hydrofoil surface were presented for analysis of the operating performance of the semi-passive system. The optimal moment of inertia and location of the activated pitching pivot were obtained. In the numerical study, the peak power coefficient and efficiency were achieved at 0.75 and 0.33, respectively.