To improve the energy extraction efficiency of oscillating hydrofoils, the influence of the gap ratio between the main and tail wings of flap type oscillating hydrofoils on their performance was explored. Computational fluid dynamics was used to numerically simulate the original oscillating hydrofoil, and the accuracy of the simulation method was verified by comparing it with publicly available data, ensuring the reliability and precision of the simulation results. In addition, a flap type oscillating hydrofoil model was constructed, and overlapping grid technology was used to conduct detailed numerical simulation studies on the clearance ratio during the flap swing process of the flap type oscillating hydrofoil with dynamic motion characteristics. The numerical simulation results reveal the significant impact mechanism of the gap ratio between the main tail fins on energy extraction efficiency. Through parametric analysis, under specific conditions of , the energy extraction efficiency of the flap type oscillating hydrofoil can reach 50.50%, an increase of 16.94% compared to the original oscillating hydrofoil efficiency. The simulation results show that the energy extraction efficiency of the flap type oscillating hydrofoil increases with the decrease of the clearance ratio.