In this paper, we study the floating breakwater composed of two pontoons and different types of plates by the 2D model tests, discuss the influence of the horizontal plates and vertical plates on the wave attenuation performance, and analyze wave attenuation performance and mechanism of the floating breakwater composed of two pontoons and the horizontal plates. The results show that for the floating breakwater composed of two pontoons and the horizontal plates, the wave attenuation performance of the slotted horizontal plates is better as a whole, especially for conditions of relatively long wave periods. The wave transmission coefficients decrease significantly and the wave attenuation performance improves significantly with the increase of the depth of the horizontal plates under water. However, compared to the twin-pontoon floating breakwater without plates, the wave attenuation performance of the floating breakwater with the horizontal plates still doesn’ t improve significantly as a whole. Compared to the twin-pontoon floating breakwater without plates, the wave transmission coefficients of the new floating breakwater with the porous vertical plates decrease significantly, and the wave attenuation performance increase significantly. Compared to the twin-pontoon floating breakwater without plates, the wave reflection coefficients of the new floating breakwater with the porous vertical plates all decrease and the wave energy damping coefficients all increase. The main reason for the improvement of the wave attenuation performance of the new floating breakwater with the porous vertical plates is that the porous vertical plates between the two pontoons can enhance the wave energy damping significantly.