In this paper, a modified model is proposed for an oscillating foil energy harvesting device, and the corresponding mathematical model is established too. A grid model for foil NACA0015 is built by using dynamic and moving mesh technology of the Computational Fluid Dynamics (CFD) software FLUENT. To understand the hydrodynamic performance and energy extraction capability of the modified model, the effects of motion parameters (heaving component parameters and pitching component parameters) are investigated. The evolutions of angle of attack and vortex field are examined. The results show that motion radius and heaving amplitude play important roles in impacting the time-averaged power coefficient. As the frequency increases, the peak value of the effective angle of attack is decline. The effect of pitching amplitude is gradually increased on the time-averaged power coefficient. Under the large frequency, the energy extraction efficiency is more sensitive to the motion radius and heaving amplitude. Moreover, there exists an optimal oscillation frequency to achieve a maximum time-averaged power coefficient for each pitching amplitude.