Abstract: Currently, industry X-ray digital radiography system based on the amorphous silicon X-ray flat-panel detector is the latest test technology. The system transfer function, namely point spread-function (PSF) consists of system geometry dispersion caused by nonpiont-source, scintillator screen dispersion and aperture sampling of detector pixels. The intensity distribution of X-ray penetration field was analyzed at each segment. The mathematical model of system PSF and modulation transfer function (MTF) was established by means of Gassian function which is the mathematic expression to describe the transfer behavior of each compose segment of the system PSF. So the system approximating effective bandwidth was achieved from half-wave width of the system PSF. In the case of the digital radiography system based on PaxScan2520 flat-panel detector; the experiment which validates the mathematical model is simply appropriate to estimate resoluble minimum detail of work-piece tested by the system and provide theoretic foundation for industry X-ray radiography.