The aim of this paper is to experimentally and numerically study a coupled-pitching hydrofoil under the fully activated mode. In the experimental study, the operating zones for the energy-releasing state and the energy-capturing state were identified. The phase relationship between the hydrodynamic torque and the angular velocity, which is determined by the combination of the primary and secondary pitching amplitudes, plays a key role in the switch of the energy releasing/capturing states. The primary pitching motion dominates the energy conversion of the hydrofoil under the fully-activated mode. The peak values of the averaged power coefficient and energy converting efficiency are achieved at 1.17 and 0.40, respectively. The numerical study found that the effective angle of attack determines the generation of the leading-edge and trailing-edge vortexes, and in turn affects the pressure distribution of the hydrofoil surface and torque generation. Compared to the traditional heaving-pitching mode, the coupled-pitching mode is horizontally affected by the negative torque because of the restriction of the primary pitching system, resulting in a lower energy capturing efficiency.