In an optical image-formation system with very narrow depth of field,it is impossible to get a very clear image because of the longitudinal distribution of different illuminant plane.The image-formation system adopting X-ray excitated glass scintillator features this character. In glass scintillator,luminous intensity of different planes decay exponentially with the depth X-ray penetrate through.In order to get a clear image from such a system,changing law of image definition is simulated in the way of geometrical optics and some very meaningful results are derived.The reasons how to adjust such a system according to the X-ray energy and the width of the glass are also explained.The experimental results according with the simulated image definition are displayed.