Based on small amplitude wave diffraction theory in two layer sea, the analytical solutions to the diffracted water wave potentials from a vertical arc-shaped porous breakwater are derived by applying eigenfunction expansion approach. According to the analytical solution, the wave loads caused by internal wave and surface wave and the distribution of maximum wave free surfaces are respectively evaluated. The results show that the appropriate hole arrangement on the surface of breakwater not only leads to an obvious reduction in surface wave and internal wave loads, but also causes changes of maximum wave distribution of diffracted water waves. The dimensionless maximum wave loads caused by internal wave may have same order of magnitude as that caused by surface wave，which implies that inner wave effects cannot be ignored. Furthermore, the relative variations of sea and water wave condition and breakwater geometry condition of breakwater have certain influence on wave diffraction, and there are some differences between surface wave and internal wave in the law of wave action.