Abstract: The focal spot size of X-ray tube has great impact on the image quality in radiographic testing. Typical EN 12543 standard gives five measuring methods (scanning method, pinhole camera radiographic method, slit camera radiographic method, edge method and measurement of the effective focal spot size of mini and micro focus Xray tubes) for focal spot size of industrial Xray systems. The testing conditions and equipment of these methods are complicated except the edge method. However, the edge method cannot be used for an absolute measurement of the focal spot. It is only useful for the checking of focal spot under field condition in order to find change of the focal spot. Based on the edge method of EN 12543, an innovative method called cylinder method for the absolute measurement was proposed in this paper. This method was different from traditional measurement of the focal spot perpendicular to X-ray emission direction called nominal focus. It could measure the focal spot size at arbitrary angle within the field angle of radiation beam by using shift-rotation mechanism. In this experiment, the focal spot size of MXR-225HP/11 was measured by double tungsten alloy cylinder and 100 μm pixel areaarray detector. Double tungsten alloy cylinder could reduce the influence of scattering and the spacing between two cylinders was designed to be variable in preparation for future research on the silt camera radiographic method. Pixel areaarray detector overcame the shortcomings of film photography and was convenient for computer image processing. In order to improve the accuracy of measurement, a calibration method for measuring the distance parameter from the focal spot to the shiftrotation mechanism and location parameter at arbitrary angle was proposed. By measuring the large and small focal spot size of MXR225HP/11 at different angles in the width and length directions, it is found that the focal spot size varies with angle and nominal focus sizes agree well with those measured by pinhole camera radiographic method provided in the manual. However, the variation law of effective focal spot size with angle in the width direction is quite different from the theoretical analysis results, and the reasons need to be further explored. The results show this method can accurately measure the nominal focus size but also the focal spot size variation within the field angle of radiation beam which will provide reference for the design of focal spot and performance evaluation of Xray tube. Besides, the layout of imaging system can be optimized by the variation of focal spot size to ensure high imaging quality.