A strong pressure increase is produced by X-ray heating a foam-solid target in comparison with heating directly the solid target, due to impedance mismatch at the foam-solid interface. We study the pressure increase of CH foam-aluminum target, and find that only by shock present in foam can the ablative pressure in aluminum increase. In this paper an analytical model based on strong wave assumption is developed to study the shock impedance mismatching crossing the interface and the interaction of rarefaction with shock. We evaluate the shock pressure amplified in the aluminum layer as a function of density and adiabatic exponent, and show that the amplified pressure is a short high pressure pulse which is destroyed by rarefactions. This is in agreement with simulation results by MULTI code.