This paper presents a mathematical model to predict the dynamic response of a lab-scale flapping-type tidal energy harvester. In the model, the forces produced by the hydroplane were derived based on a quasi-steady-state assumption, and no approximation was applied to the terms of the force equation. Therefore, the equation of motion becomes nonlinear; it was solved using the Runge–Kutta method. The mathematical dynamic model was validated through a series of experiments. Thereafter, the flapping speed of the system was estimated and compared with the measured speed. The comparison proved that the dynamic model reasonably estimates the flapping response, flapping speed, and generated forces. The dynamic model can be used for performance prediction of the lab-scale flapping tidal energy harvester and optimization of the scaled-up model in the future.