The interaction processes of bremsstrahlung X rays with CsI:Na scintillator of different thickness(200~400 μm) are first simulated by means of Monte Carlo analysis. The energy spectra of exit photons from CsI:Na are presented, which are used as the incident spectra on iron, the constituents of the inner electrodes and shell of intensifiers. The backscattering properties of X rays in intensifiers are analyzed. The potential producing bremsstrahlung X rays is 20~120 kV. Results show that with the increase of X ray tube potential the number of backscattered photons gets bigger and their mean energies increase rapidly.