The characteristics of a computerized radiography system were briefly discussed. The method for X－ray image digitalization was introduced. The hardware circuit for X－ray image data acquistion was given. Architecture of Windows NT and its kernal mode driver were briefly illustrated, and the working principle of I/O request in Windows NT was interpreted. The kernel mode driver for the X－ray image data acquisition card was designed with NT DDK. The real－time acquisition and storing of X－ray image data were carried out by the combination of hardware and software. In the long－term experiment, it has been proved that the method is feasible and reliable, and it can meet the practical commands.