This paper aims to study the influence of motion parameters on the power-extraction efficiency of oscillating hydrofoils in tidal current, investigate the distributions of the motion parameters of hydrofoils at high power-extraction efficiency, and improve the power-extraction efficiency of oscillating hydrofoils in tidal power-extraction devices. A numerical calculation model of oscillating hydrofoils was established based on the overset grid technique, and the effectiveness of the grid was verified. By modifying the motion parameters of a hydrofoil including reduced frequency f^*, pitching amplitude θ₀, and heave amplitude h₀, the hydrodynamic characteristics of two-dimensional oscillating hydrofoil and the corresponding power-extraction efficiency were investigated. In addition, the influence of vortex shedding on the power-extraction efficiency during the motion of the hydrofoil was analyzed. The pressure distribution when the wing-in-ground effect was formed between two hydrofoils was also studied. The hydrodynamic characteristics of the hydrofoil and the influence of the wing-in-ground effect on the power-extraction efficiency were analyzed under this circumstance. Results show that reasonable planning of the reduced frequency, pitching amplitude, heaving amplitude, and shaft position of the oscillating hydrofoil could all improve the power-extraction efficiency. The generation of vortex shedding during the oscillating motion of the hydrofoil could affect the power-extraction efficiency. The wing-in-ground effect between the parallel symmetrical double hydrofoils would form a high-pressure area in the flow field between the two hydrofoils and increase the lift of the hydrofoils. Increasing the lift-to-drag ratio was beneficial to improve the power-extraction efficiency. When the minimum distance between the two hydrofoils reduced, the power-extraction efficiency of the hydrofoil increased due to the enhancement of ground effect.