An advanced oscillating-water-column-type (OWC-type) breakwater with a horizontal underneath plate was proposed. The hydrodynamic characteristics of the system under three different plate breadths and twelve different wave periods were studied using two-dimensional laboratory experiments. The effect of wave height on the hydrodynamic parameters of the breakwater model was explored by experiments. The experimental results show that the wave-damping ability of the breakwater was improved by setting the underneath plate reasonably: the transmission coefficient for incident wave periods within 1.6 s was less than 0.5, as the horizontal bottom plate width was equal to the width between the model plates; the wave transmission was less than 50% for incident wave periods within 2.2 s, as the horizontal bottom width was increased to twice the width between the model plates. The presence of the underneath plate is beneficial for the energy dissipation, which particularly improves the performance of the breakwater for the long-wave regimes. The change of wave height has little effect on the relevant hydrodynamic parameters.