A broad angular response supermirror is designed by the simplex optimization method and fabricated by dc magnetron sputtering. The negative effect of the interfacial imperfection, mainly consulting from interface roughness and diffusion, is emerged in the calculation of the precise performance of the supermirror. The reflectivity of such a supermirror is measured by the x-ray diffraction instrument (XRD) at Cu Kα line (λ = 0.154 nm). The experimental reflectivity is about 30% in a fixed broad grazing incident angular range (0.55°-0.85°). The fitting data prove that the thickness of each layer, which is larger than the prospect 0.5nm, is different from the designed one and the roughness in the supermirror is about 0.85nm.