An X-ray detector project,based on the narrow CsI(Tl) scintillating crystal and area-array CCD,using fiber optic and fiber optic plate as light coupled and transmission with fan-beam linear array scanning,has been put forth in this paper. Below 450 keV for x-ray energy,on the base of the physical analyzing and mathematical modeling,the luminescent efficiency property of narrow CsI(Tl)scintillation crystal has been studied by using Matlab. While the parameters of μ_ph、μ_c、R、R_s、and σ equal to 0.00313,0.0000295,0.95,0.8,and 0.000222 μm^－1respectively,it is concluded that the length range of the narrow CsI(Tl) scintillation crystal is 926～4512 μm,the height range is 242～5000 μm,and the width is 242～5000 μm. In the range of the above size,it can be obtained that space(or geometry) resolution of the narrow CsI(Tl) scintillation crystal is better and luminescent efficiency is also the highest.