The reflection of small amplitude water waves impinging on a double slit-wall breakwater is numerically studied by applying the Boundary Integral Method (BIM). The depth of wave chambers can be changed from the one outside of the chamber in the present numerical analysis. The energy loss due to flow separation near slits is modeled by introducing a complex blockage coefficient. Comparison of numerical results with experimental data shows the validity of the present analysis. Using the present numerical scheme, a favorable depth of wave chambers and a favorable position of second slit wall in the wave chamber are studied and discussed. A comparison with a single slit-wall breakwater in reflection coefficient is also made.