Oscillating underwater flexible structure actuated by smart materials are widely used in the fields of robotic fish， autono? mous underwater vehicle， precision medical instrument， and so on. In this paper， the nonlinear hydrodynamics of an underwater Macro Fiber Composite （MFC）-actuated flexible cantilever undergoing large amplitude vibration is studied. The fluid-structure coupled dynamic equation of the proposed structure is established. Parametric 2D CFD studies of the proposed structure at different characteristic frequencies and amplitudes are performed. The distribution and evolution of the flow field in the vicinity of the vibrat? ing structure are revealed. CFD results show that the vortex shedding， diffusion and convection phenomena which are responsible for the nonlinear hydrodynamic damping effect appear as the vibration amplitude increases. Then， a manageable expression for the revised hydrodynamic function governed by the interplay of the characteristic frequency and vibration amplitude is presented to mod? el the hydrodynamic load exerted on the flexible structure undergoing finite amplitude vibration. The imaginary part of the revised hydrodynamic function which accounts for the hydrodynamic damping effect decreases with the characteristic frequency for the small amplitude vibration. It first decreases then increases for the finite amplitude vibration， exhibiting a strong nonlinear behavior. Experimental results show that the measured frequency response spectrums of the proposed structure undergoing finite amplitude match well with the predicted results of the developed model. Thus， the validities of the developed hydrodynamic function and flu? id-structure coupled dynamic equation are demonstrated.