Abstract: Xray is always returned to visible light for detection by scintillator in high energy radiography systems. The system spatial resolution is affected by scintillator thickness, material and energy of the input X photons. This influence is analyzed theoretically and the calculating model is built. For providing the parameters to optimize the design of the system, the relation between the scintillator spatial resolution and the scintillator thickness, material and energy of the input X photon is calculated. The increasing of scintillator thickness will reduce the spatial resolution while the increasing of input X photon energy will raise it. The spatial resolution of BGO is higher than that of CsI:Tl with same thickness.