The generators using oscillating foil to extract tidal energy have obvious advantages in shallow water. To get rid of the complicated control system, we proposed a hydraulic system to couple dual foils and realize the self-sustained oscillation motion of the foil. The coupling equations related to the foils' motion and hydrodynamic are established. Computations are performed using the software Fluent with a NACA0015 foil. A User Defined Function is compiled for iteration. The classical Runge-Kutta method is employed to compute the displacement and moving velocity. Computation results demonstrate that the hydraulic coupling system realizes well the self-sustained oscillation motion of foil instead of external control. The response of the coupling system is independent of the initial pitching angle. The damping coefficients have a significant effect on the response of coupling system. The response of coupling system, oscillation amplitude and frequency, declines with the increasing of damping coefficient, especially at relatively low damping coefficient. The energy consumption of pitching motion most appears in the stage of angle decrease. The evolutions of time-averaged power coefficient and energy harvesting efficiency versus damping coefficient are quite similar to that of the oscillation amplitude and frequency. (C) 2017 Elsevier Ltd. All rights reserved.