The numerical simulation model of win pontoon-twin horizontal plate floating breakwater is established by hydrodynamics simulation software ANSYSWorkbench and AQWA Graphical Supervisor. The investigation of the floating breakwater including the underwater penetration and the spacing of two pontoons and two plates shows that with the increase of the underwater penetration and the spacing of two plates the transmission coefficient decreases. When the relative spacing of two pontoons varies from 0.22 to 0.33,the transmission coefficients turn smaller. The natural period of the twin pontoon-twin horizontal plate floating breakwater is about 4.8～5.3 s. In the design,we should pay attention to avoid producing resonance when the natural period of floating breakwater and the wave period are close. The research will provide a theoretical basis for the breakwater design.