MFC actuators, widely used in intelligent sensing/actuation, energy harvesting, underwater bionic robots and other fields, have increasingly attracted attention due to their good flexibility, large actuation force and excellent waterproof performance. In this paper, the vibration characteristics and dynamic response of underwater flexible structure driven by MFC actuators were studied. The driving force of the MFC actuators were calculated, and the additional inertia force and additional damping force of the fluid were derived according to Morrison's semi-empirical formula. Based on the Euler-Bernoulli beam theory, the assumed mode method and the second kind of Lagrange equation, a coupled nonlinear dynamic model of MFC-actuated underwater flexible structures was established. The harmonic balance method was used to convert the nonlinear damping into linear. The numerical simulation and experimental results show that the local stiffness of the flexible beam structure with the MFC actuator is increased, and the measured modal shape is basically consistent with the simulated one. Affected by the hydrodynamic force of the surrounding fluid, the first two orders resonance frequency and dynamic response of the MFC-actuated cantilever beam underwater decrease significantly. The amplitude-frequency response curve predicted by the model is in good agreement with the measured curve, which confirms the validity of the coupling dynamic model. This study provides a reference for underwater bionic propulsion devices based on smart materials.