A flat-response X-ray photocathode at the energy range of 0.1~5 keV was presented. The photocathode consists of two layers of metal material with same area and different thickness. After optimizing the thickness and microstructure of two metal layers, the proposed photocathode does not need complex corrections in the interested X-ray energy range of inertial confinement fusion experiment due to the flat-response characteristics. The flatness of the proposed X-ray photocathode with 4 kinds of materials respectively was simulated by adjusting the material, thickness and duty ratio. The simulation results show that the proposed photocathode has flat-response with flatness better than 5% at the X-ray energy range of 0.1~5 keV.