A beam bonded with a Macro-Fiber Composite actuator can generate transverse deformations, enabling mechanism-free flapping wings and underwater oscillating fins. A piezocomposite beam can be actuated harmonically by a resistance-inductor-capacitor resonator, and interact with ambient fluid to yield flow vectoring, control, and/or propulsion. In this paper, an electro-mechanical-fluid coupled model is developed to obtain insight into the coupling mechanism, and provide guidance to investigate the system optimal parameters. To give prominence to the coupling mechanisms, the system is simplified to a lumped parameter model. The electromechanical coupling is modeled as a transformer, and the fluid-structure interaction is represented by the added mass and added damping effects, which are determined by solving linearized Navier-Stokes equations. The coupled model is converted into a transfer function, where the system input is voltage, and the output is displacement. A case study is conducted based on the coupled model, and important system properties are revealed.